datasets 1.6 ignores cache
OK, It doesn't look like we can use the proposed workaround - see https://github.com/huggingface/transformers/issues/11801

Could you please add an env var for us to be able to turn off this unwanted in our situation behavior? It is really problematic for dev work, when one needs to restart the training very often and needs a quick startup time. Manual editing of standard scripts is not a practical option when one uses examples.

This could also be a problem for tests, which will be slower because of lack of cache, albeit usually we use tiny datasets there. I think we want caching for tests.

Thank you.
Moving from https://github.com/huggingface/transformers/issues/11801#issuecomment-845546612 

Quoting @VictorSanh:

> 
> I downgraded datasets to `1.5.0` and printed `tokenized_datasets.cache_files` (L335):
> 
> > `{'train': [{'filename': '/home/victor/.cache/huggingface/datasets/openwebtext10k/plain_text/1.0.0/3a8df094c671b4cb63ed0b41f40fb3bd855e9ce2e3765e5df50abcdfb5ec144b/cache-c6aefe81ca4e5152.arrow'}], 'validation': [{'filename': '/home/victor/.cache/huggingface/datasets/openwebtext10k/plain_text/1.0.0/3a8df094c671b4cb63ed0b41f40fb3bd855e9ce2e3765e5df50abcdfb5ec144b/cache-97cf4c813e6469c6.arrow'}]}`
> 
> while the same command with the latest version of datasets (actually starting at `1.6.0`) gives:
> > `{'train': [], 'validation': []}`
> 

I also confirm that downgrading to `datasets==1.5.0` makes things fast again - i.e. cache is used.

to reproduce:
```
USE_TF=0 python  examples/pytorch/language-modeling/run_clm.py \
    --model_name_or_path gpt2 \
    --dataset_name "stas/openwebtext-10k" \
    --output_dir output_dir \
    --overwrite_output_dir \
    --do_train \
    --do_eval \
    --max_train_samples 1000 \
    --max_eval_samples 200 \
    --per_device_train_batch_size 4 \
    --per_device_eval_batch_size 4 \
    --num_train_epochs 1 \
    --warmup_steps 8 \
    --block_size 64 \
    --fp16 \
    --report_to none
```

the first time the startup is slow and some 5 tqdm bars. It shouldn't do it on consequent runs. but with `datasets>1.5.0` it rebuilds on every run.

@lhoestq 

datasets 1.6 ignores cache 
 Moving from https://github.com/huggingface/transformers/issues/11801#issuecomment-845546612 

Quoting @VictorSanh:

> 
> I downgraded datasets to `1.5.0` and printed `tokenized_datasets.cache_files` (L335):
> 
> > `{'train': [{'filename': '/home/victor/.cache/huggingface/datasets/openwebtext10k/plain_text/1.0.0/3a8df094c671b4cb63ed0b41f40fb3bd855e9ce2e3765e5df50abcdfb5ec144b/cache-c6aefe81ca4e5152.arrow'}], 'validation': [{'filename': '/home/victor/.cache/huggingface/datasets/openwebtext10k/plain_text/1.0.0/3a8df094c671b4cb63ed0b41f40fb3bd855e9ce2e3765e5df50abcdfb5ec144b/cache-97cf4c813e6469c6.arrow'}]}`
> 
> while the same command with the latest version of datasets (actually starting at `1.6.0`) gives:
> > `{'train': [], 'validation': []}`
> 

I also confirm that downgrading to `datasets==1.5.0` makes things fast again - i.e. cache is used.

to reproduce:
```
USE_TF=0 python  examples/pytorch/language-modeling/run_clm.py \
    --model_name_or_path gpt2 \
    --dataset_name "stas/openwebtext-10k" \
    --output_dir output_dir \
    --overwrite_output_dir \
    --do_train \
    --do_eval \
    --max_train_samples 1000 \
    --max_eval_samples 200 \
    --per_device_train_batch_size 4 \
    --per_device_eval_batch_size 4 \
    --num_train_epochs 1 \
    --warmup_steps 8 \
    --block_size 64 \
    --fp16 \
    --report_to none
```

the first time the startup is slow and some 5 tqdm bars. It shouldn't do it on consequent runs. but with `datasets>1.5.0` it rebuilds on every run.

@lhoestq 
 
 OK, It doesn't look like we can use the proposed workaround - see https://github.com/huggingface/transformers/issues/11801

Could you please add an env var for us to be able to turn off this unwanted in our situation behavior? It is really problematic for dev work, when one needs to restart the training very often and needs a quick startup time. Manual editing of standard scripts is not a practical option when one uses examples.

This could also be a problem for tests, which will be slower because of lack of cache, albeit usually we use tiny datasets there. I think we want caching for tests.

Thank you.