Crash when `num_proc` > dataset length for `map()` on a `datasets.Dataset`.
Hi ! It looks like the issue comes from pyarrow. What version of pyarrow are you using ? How did you install it ?
## Describe the bug
Crash if when using `num_proc` > 1 (I used 16) for `map()` on a `datasets.Dataset`.

I believe I've had cases where `num_proc` > 1 works before, but now it seems either inconsistent, or depends on my data. I'm not sure whether the issue is on my end, because it's difficult for me to debug! Any tips greatly appreciated, I'm happy to provide more info if it would helps us diagnose.

## Steps to reproduce the bug
```python
# this function will be applied with map()
def tokenize_function(examples):
    return tokenizer(
        examples["text"],
        padding=PaddingStrategy.DO_NOT_PAD,
        truncation=True,
    )

# data_files is a Dict[str, str] mapping name -> path
datasets = load_dataset("text", data_files={...})  

# this is where the error happens if num_proc = 16,
# but is fine if num_proc = 1
tokenized_datasets = datasets.map(
    tokenize_function,
    batched=True,
    num_proc=num_workers,
)
```

## Expected results
The `map()` function succeeds with `num_proc` > 1.

## Actual results
![image](https://user-images.githubusercontent.com/1170062/121404271-a6cc5200-c910-11eb-8e27-5c893bd04042.png)
![image](https://user-images.githubusercontent.com/1170062/121404362-be0b3f80-c910-11eb-9117-658943029aef.png)

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.6.2
- Platform: Linux-5.4.0-73-generic-x86_64-with-glibc2.31
- Python version: 3.9.5
- PyTorch version (GPU?): 1.8.1+cu111 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: Yes, but I think N/A for this issue
- Using distributed or parallel set-up in script?: Multi-GPU on one machine, but I think also N/A for this issue

Crash when `num_proc` > dataset length for `map()` on a `datasets.Dataset`. 
 ## Describe the bug
Crash if when using `num_proc` > 1 (I used 16) for `map()` on a `datasets.Dataset`.

I believe I've had cases where `num_proc` > 1 works before, but now it seems either inconsistent, or depends on my data. I'm not sure whether the issue is on my end, because it's difficult for me to debug! Any tips greatly appreciated, I'm happy to provide more info if it would helps us diagnose.

## Steps to reproduce the bug
```python
# this function will be applied with map()
def tokenize_function(examples):
    return tokenizer(
        examples["text"],
        padding=PaddingStrategy.DO_NOT_PAD,
        truncation=True,
    )

# data_files is a Dict[str, str] mapping name -> path
datasets = load_dataset("text", data_files={...})  

# this is where the error happens if num_proc = 16,
# but is fine if num_proc = 1
tokenized_datasets = datasets.map(
    tokenize_function,
    batched=True,
    num_proc=num_workers,
)
```

## Expected results
The `map()` function succeeds with `num_proc` > 1.

## Actual results
![image](https://user-images.githubusercontent.com/1170062/121404271-a6cc5200-c910-11eb-8e27-5c893bd04042.png)
![image](https://user-images.githubusercontent.com/1170062/121404362-be0b3f80-c910-11eb-9117-658943029aef.png)

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.6.2
- Platform: Linux-5.4.0-73-generic-x86_64-with-glibc2.31
- Python version: 3.9.5
- PyTorch version (GPU?): 1.8.1+cu111 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: Yes, but I think N/A for this issue
- Using distributed or parallel set-up in script?: Multi-GPU on one machine, but I think also N/A for this issue
 
 Hi ! It looks like the issue comes from pyarrow. What version of pyarrow are you using ? How did you install it ?