File exists error when used with TPU
Here's a minimal working example to reproduce this issue.

Assumption:
- You have access to TPU.
- You have installed `transformers` and `nlp`.
- You have tokenizer files (`config.json`, `merges.txt`, `vocab.json`) under the directory named `model_name`.
- You have `xla_spawn.py` (Download from https://github.com/huggingface/transformers/blob/master/examples/xla_spawn.py).
- You have saved the following script as `prepare_cached_dataset.py`.

```python
#!/usr/bin/env python3
import argparse
from transformers import AutoTokenizer
from nlp import load_dataset

def main():
    parser = argparse.ArgumentParser(description='description')
    parser.add_argument('--tokenizer_name', type=str, help='Pretrained tokenizer name or path if not the same as model_name')
    parser.add_argument('--data_file', type=str, help='The input data file (a text file).')
    parser.add_argument('--cache_dir', type=str, default=None, help='Where do you want to store the pretrained models downloaded from s3')
    parser.add_argument('--block_size', type=int, default=-1, help='The training dataset will be truncated in block of this size for training')
    parser.add_argument('--tpu_num_cores', type=int, default=1, help='Number of TPU cores to use (1 or 8). For xla_apwan.py')
    model_args = parser.parse_args()
    
    tokenizer = AutoTokenizer.from_pretrained(model_args.tokenizer_name, cache_dir=model_args.cache_dir, use_fast=True)
    
    dataset = load_dataset("text", data_files=model_args.data_file, split="train")
    dataset = dataset.map(lambda ex: tokenizer(ex["text"], add_special_tokens=True,
                                truncation=True, max_length=model_args.block_size), batched=True)
    dataset.set_format(type='torch', columns=['input_ids'])

def _mp_fn(index):
    # For xla_spawn (TPUs)
    main()

if __name__ == "__main__":
    main()
```

- Run the following command. Replace `your_training_data` with some text file.

```bash
export TRAIN_DATA=your_training_data

python prepare_cached_dataset.py \
--tokenizer_name=model_name \
--block_size=512 \
--data_file=$TRAIN_DATA
```
- Check the cached directory.
```bash
ls -lha /home/*****/.cache/huggingface/datasets/text/default-e84dd29acc4ad9ef/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d
total 132M
drwxr-xr-x 2 ***** ***** 4.0K Aug 28 13:08 .
drwxr-xr-x 3 ***** ***** 4.0K Aug 28 13:08 ..
-rw------- 1 ***** *****  99M Aug 28 13:08 cache-bfc7cb0702426d19242db5e8c079f04b.arrow
-rw-r--r-- 1 ***** *****  670 Aug 28 13:08 dataset_info.json
-rw-r--r-- 1 ***** *****    0 Aug 28 13:08 LICENSE
-rw-r--r-- 1 ***** *****  33M Aug 28 13:08 text-train.arrow
```

- Run the same script again. (The output should be just `Using custom data configuration default`.)
```
python prepare_cached_dataset.py \
--tokenizer_name=model_name \
--block_size=512 \
--data_file=$TRAIN_DATA
```
- Check the cached directory.
```bash
ls -lha /home/*****/.cache/huggingface/datasets/text/default-e84dd29acc4ad9ef/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d
total 132M
drwxr-xr-x 2 ***** ***** 4.0K Aug 28 13:08 .
drwxr-xr-x 3 ***** ***** 4.0K Aug 28 13:08 ..
-rw------- 1 ***** *****  99M Aug 28 13:08 cache-bfc7cb0702426d19242db5e8c079f04b.arrow
-rw-r--r-- 1 ***** *****  670 Aug 28 13:20 dataset_info.json
-rw-r--r-- 1 ***** *****    0 Aug 28 13:20 LICENSE
-rw-r--r-- 1 ***** *****  33M Aug 28 13:08 text-train.arrow
```
- The cached file (`cache-bfc7cb0702426d19242db5e8c079f04b.arrow`) is reused.
- Now, run this script with `xla_spawn.py`. Ideally, it should reuse the cached file, however, you will see each process is creating a cache file again.

```bash
python xla_spawn.py --num_cores 8 \
prepare_cached_dataset.py \
--tokenizer_name=model_name \
--block_size=512 \
--data_file=$TRAIN_DATA
```

- Check the cached directory. There are two arrrow files.
```bash
ls -lha /home/*****/.cache/huggingface/datasets/text/default-e84dd29acc4ad9ef/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d
total 230M
drwxr-xr-x 2 ***** ***** 4.0K Aug 28 13:25 .
drwxr-xr-x 3 ***** ***** 4.0K Aug 28 13:08 ..
-rw------- 1 ***** *****  99M Aug 28 13:08 cache-bfc7cb0702426d19242db5e8c079f04b.arrow
-rw------- 1 ***** *****  99M Aug 28 13:25 cache-e0e2313e49c8a110aafcc8133154c19a.arrow
-rw-r--r-- 1 ***** *****  670 Aug 28 13:24 dataset_info.json
-rw-r--r-- 1 ***** *****    0 Aug 28 13:24 LICENSE
-rw-r--r-- 1 ***** *****  33M Aug 28 13:08 text-train.arrow
```

Hi,

I'm getting a "File exists" error when I use [text dataset](https://github.com/huggingface/nlp/tree/master/datasets/text) for pre-training a RoBERTa model using `transformers` (3.0.2) and `nlp`(0.4.0) on a VM with TPU (v3-8).

I modified [line 131 in the original `run_language_modeling.py`](https://github.com/huggingface/transformers/blob/master/examples/language-modeling/run_language_modeling.py#L131) as follows:

```python
# line 131: return LineByLineTextDataset(tokenizer=tokenizer, file_path=file_path, block_size=args.block_size)
dataset = load_dataset("text", data_files=file_path, split="train")
dataset = dataset.map(lambda ex: tokenizer(ex["text"], add_special_tokens=True,
                                        truncation=True, max_length=args.block_size), batched=True)
dataset.set_format(type='torch', columns=['input_ids'])
return dataset
```

When I run this with [`xla_spawn.py`](https://github.com/huggingface/transformers/blob/master/examples/xla_spawn.py), I get the following error (it produces one message per core in TPU, which I believe is fine).

It seems the current version doesn't take into account distributed training processes as in [this example](https://github.com/huggingface/transformers/blob/a573777901e662ec2e565be312ffaeedef6effec/src/transformers/data/datasets/language_modeling.py#L35-L38)?

```
08/25/2020 13:59:41 - WARNING - nlp.builder -   Using custom data configuration default
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Exception in device=TPU:6: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Exception in device=TPU:4: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Exception in device=TPU:1: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Exception in device=TPU:7: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Exception in device=TPU:3: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Exception in device=TPU:2: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Exception in device=TPU:0: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Traceback (most recent call last):
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
Traceback (most recent call last):
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
      main()
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
      File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
Traceback (most recent call last):
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Traceback (most recent call last):
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
```


File exists error when used with TPU 
 Hi,

I'm getting a "File exists" error when I use [text dataset](https://github.com/huggingface/nlp/tree/master/datasets/text) for pre-training a RoBERTa model using `transformers` (3.0.2) and `nlp`(0.4.0) on a VM with TPU (v3-8).

I modified [line 131 in the original `run_language_modeling.py`](https://github.com/huggingface/transformers/blob/master/examples/language-modeling/run_language_modeling.py#L131) as follows:

```python
# line 131: return LineByLineTextDataset(tokenizer=tokenizer, file_path=file_path, block_size=args.block_size)
dataset = load_dataset("text", data_files=file_path, split="train")
dataset = dataset.map(lambda ex: tokenizer(ex["text"], add_special_tokens=True,
                                        truncation=True, max_length=args.block_size), batched=True)
dataset.set_format(type='torch', columns=['input_ids'])
return dataset
```

When I run this with [`xla_spawn.py`](https://github.com/huggingface/transformers/blob/master/examples/xla_spawn.py), I get the following error (it produces one message per core in TPU, which I believe is fine).

It seems the current version doesn't take into account distributed training processes as in [this example](https://github.com/huggingface/transformers/blob/a573777901e662ec2e565be312ffaeedef6effec/src/transformers/data/datasets/language_modeling.py#L35-L38)?

```
08/25/2020 13:59:41 - WARNING - nlp.builder -   Using custom data configuration default
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
08/25/2020 13:59:43 - INFO - nlp.builder -   Generating dataset text (/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d)
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Exception in device=TPU:6: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Exception in device=TPU:4: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Exception in device=TPU:1: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Exception in device=TPU:7: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Exception in device=TPU:3: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Downloading and preparing dataset text/default-b0932b2bdbb63283 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/
447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d...
Exception in device=TPU:2: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Exception in device=TPU:0: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Traceback (most recent call last):
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
Traceback (most recent call last):
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
      main()
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
      File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
Traceback (most recent call last):
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
Traceback (most recent call last):
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 231, in _start_fn
    fn(gindex, *args)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 300, in _mp_fn
    main()
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 240, in main
    train_dataset = get_dataset(data_args, tokenizer=tokenizer) if training_args.do_train else None
  File "/home/*****/huggingface_roberta/run_language_modeling.py", line 134, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split="train")
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/load.py", line 546, in load_dataset
    download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 450, in download_and_prepare
    with incomplete_dir(self._cache_dir) as tmp_data_dir:
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/site-packages/nlp/builder.py", line 422, in incomplete_dir
    os.makedirs(tmp_dir)
  File "/anaconda3/envs/torch-xla-1.6/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
FileExistsError: [Errno 17] File exists: '/home/*****/.cache/huggingface/datasets/text/default-b0932b2bdbb63283/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d.incomplete'
```

 
 Here's a minimal working example to reproduce this issue.

Assumption:
- You have access to TPU.
- You have installed `transformers` and `nlp`.
- You have tokenizer files (`config.json`, `merges.txt`, `vocab.json`) under the directory named `model_name`.
- You have `xla_spawn.py` (Download from https://github.com/huggingface/transformers/blob/master/examples/xla_spawn.py).
- You have saved the following script as `prepare_cached_dataset.py`.

```python
#!/usr/bin/env python3
import argparse
from transformers import AutoTokenizer
from nlp import load_dataset

def main():
    parser = argparse.ArgumentParser(description='description')
    parser.add_argument('--tokenizer_name', type=str, help='Pretrained tokenizer name or path if not the same as model_name')
    parser.add_argument('--data_file', type=str, help='The input data file (a text file).')
    parser.add_argument('--cache_dir', type=str, default=None, help='Where do you want to store the pretrained models downloaded from s3')
    parser.add_argument('--block_size', type=int, default=-1, help='The training dataset will be truncated in block of this size for training')
    parser.add_argument('--tpu_num_cores', type=int, default=1, help='Number of TPU cores to use (1 or 8). For xla_apwan.py')
    model_args = parser.parse_args()
    
    tokenizer = AutoTokenizer.from_pretrained(model_args.tokenizer_name, cache_dir=model_args.cache_dir, use_fast=True)
    
    dataset = load_dataset("text", data_files=model_args.data_file, split="train")
    dataset = dataset.map(lambda ex: tokenizer(ex["text"], add_special_tokens=True,
                                truncation=True, max_length=model_args.block_size), batched=True)
    dataset.set_format(type='torch', columns=['input_ids'])

def _mp_fn(index):
    # For xla_spawn (TPUs)
    main()

if __name__ == "__main__":
    main()
```

- Run the following command. Replace `your_training_data` with some text file.

```bash
export TRAIN_DATA=your_training_data

python prepare_cached_dataset.py \
--tokenizer_name=model_name \
--block_size=512 \
--data_file=$TRAIN_DATA
```
- Check the cached directory.
```bash
ls -lha /home/*****/.cache/huggingface/datasets/text/default-e84dd29acc4ad9ef/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d
total 132M
drwxr-xr-x 2 ***** ***** 4.0K Aug 28 13:08 .
drwxr-xr-x 3 ***** ***** 4.0K Aug 28 13:08 ..
-rw------- 1 ***** *****  99M Aug 28 13:08 cache-bfc7cb0702426d19242db5e8c079f04b.arrow
-rw-r--r-- 1 ***** *****  670 Aug 28 13:08 dataset_info.json
-rw-r--r-- 1 ***** *****    0 Aug 28 13:08 LICENSE
-rw-r--r-- 1 ***** *****  33M Aug 28 13:08 text-train.arrow
```

- Run the same script again. (The output should be just `Using custom data configuration default`.)
```
python prepare_cached_dataset.py \
--tokenizer_name=model_name \
--block_size=512 \
--data_file=$TRAIN_DATA
```
- Check the cached directory.
```bash
ls -lha /home/*****/.cache/huggingface/datasets/text/default-e84dd29acc4ad9ef/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d
total 132M
drwxr-xr-x 2 ***** ***** 4.0K Aug 28 13:08 .
drwxr-xr-x 3 ***** ***** 4.0K Aug 28 13:08 ..
-rw------- 1 ***** *****  99M Aug 28 13:08 cache-bfc7cb0702426d19242db5e8c079f04b.arrow
-rw-r--r-- 1 ***** *****  670 Aug 28 13:20 dataset_info.json
-rw-r--r-- 1 ***** *****    0 Aug 28 13:20 LICENSE
-rw-r--r-- 1 ***** *****  33M Aug 28 13:08 text-train.arrow
```
- The cached file (`cache-bfc7cb0702426d19242db5e8c079f04b.arrow`) is reused.
- Now, run this script with `xla_spawn.py`. Ideally, it should reuse the cached file, however, you will see each process is creating a cache file again.

```bash
python xla_spawn.py --num_cores 8 \
prepare_cached_dataset.py \
--tokenizer_name=model_name \
--block_size=512 \
--data_file=$TRAIN_DATA
```

- Check the cached directory. There are two arrrow files.
```bash
ls -lha /home/*****/.cache/huggingface/datasets/text/default-e84dd29acc4ad9ef/0.0.0/447f2bcfa2a721a37bc8fdf23800eade1523cf07f7eada6fe661fe4d070d380d
total 230M
drwxr-xr-x 2 ***** ***** 4.0K Aug 28 13:25 .
drwxr-xr-x 3 ***** ***** 4.0K Aug 28 13:08 ..
-rw------- 1 ***** *****  99M Aug 28 13:08 cache-bfc7cb0702426d19242db5e8c079f04b.arrow
-rw------- 1 ***** *****  99M Aug 28 13:25 cache-e0e2313e49c8a110aafcc8133154c19a.arrow
-rw-r--r-- 1 ***** *****  670 Aug 28 13:24 dataset_info.json
-rw-r--r-- 1 ***** *****    0 Aug 28 13:24 LICENSE
-rw-r--r-- 1 ***** *****  33M Aug 28 13:08 text-train.arrow
```
