Load text file for RoBERTa pre-training.  
 I migrate my question from https://github.com/huggingface/transformers/pull/4009#issuecomment-690039444

I tried to train a Roberta from scratch using transformers. But I got OOM issues with loading a large text file. 
According to the suggestion from @thomwolf , I tried to implement `datasets` to load my text file. This test.txt is a simple sample where each line is a sentence.
```
from datasets import load_dataset
dataset = load_dataset('text', data_files='test.txt',cache_dir="./")
dataset.set_format(type='torch',columns=["text"])
dataloader = torch.utils.data.DataLoader(dataset, batch_size=8)
next(iter(dataloader))
```

But dataload cannot yield sample and error is:
```
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-12-388aca337e2f> in <module>
----> 1 next(iter(dataloader))

/Library/Python/3.7/site-packages/torch/utils/data/dataloader.py in __next__(self)
    361 
    362     def __next__(self):
--> 363         data = self._next_data()
    364         self._num_yielded += 1
    365         if self._dataset_kind == _DatasetKind.Iterable and \

/Library/Python/3.7/site-packages/torch/utils/data/dataloader.py in _next_data(self)
    401     def _next_data(self):
    402         index = self._next_index()  # may raise StopIteration
--> 403         data = self._dataset_fetcher.fetch(index)  # may raise StopIteration
    404         if self._pin_memory:
    405             data = _utils.pin_memory.pin_memory(data)

/Library/Python/3.7/site-packages/torch/utils/data/_utils/fetch.py in fetch(self, possibly_batched_index)
     42     def fetch(self, possibly_batched_index):
     43         if self.auto_collation:
---> 44             data = [self.dataset[idx] for idx in possibly_batched_index]
     45         else:
     46             data = self.dataset[possibly_batched_index]

/Library/Python/3.7/site-packages/torch/utils/data/_utils/fetch.py in <listcomp>(.0)
     42     def fetch(self, possibly_batched_index):
     43         if self.auto_collation:
---> 44             data = [self.dataset[idx] for idx in possibly_batched_index]
     45         else:
     46             data = self.dataset[possibly_batched_index]

KeyError: 0
```

`dataset.set_format(type='torch',columns=["text"])` returns a log says:
```
Set __getitem__(key) output type to torch for ['text'] columns (when key is int or slice) and don't output other (un-formatted) columns.
```

I noticed the dataset is `DatasetDict({'train': Dataset(features: {'text': Value(dtype='string', id=None)}, num_rows: 44)})`.
Each sample can be accessed by `dataset["train"]["text"]` instead of `dataset["text"]`. 

Could you please give me any suggestions on how to modify this code to load the text file?

Versions:
Python version 3.7.3
PyTorch version 1.6.0 
TensorFlow version 2.3.0 
datasets version: 1.0.1 
 I solved the above issue by downloading text.py manually and passing the path to the `load_dataset` function. 

Now, I have a new issue with the Read-only file system.

The error is: 
```
I0916 22:14:38.453380 140737353971520 filelock.py:274] Lock 140734268996072 acquired on /scratch/chiyuzh/roberta/text.py.lock
Found main folder for dataset /scratch/chiyuzh/roberta/text.py at /home/chiyuzh/.cache/huggingface/modules/datasets_modules/datasets/text
Creating specific version folder for dataset /scratch/chiyuzh/roberta/text.py at /home/chiyuzh/.cache/huggingface/modules/datasets_modules/datasets/text/512f465342e4f4cd07a8791428a629c043bb89d55ad7817cbf7fcc649178b014
I0916 22:14:38.530371 140737353971520 filelock.py:318] Lock 140734268996072 released on /scratch/chiyuzh/roberta/text.py.lock
Traceback (most recent call last):
  File "/scratch/chiyuzh/roberta/run_language_modeling_hg.py", line 1019, in <module>
    main()
  File "/scratch/chiyuzh/roberta/run_language_modeling_hg.py", line 962, in main
    train_dataset = load_and_cache_examples(args, tokenizer, evaluate=False)
  File "/scratch/chiyuzh/roberta/run_language_modeling_hg.py", line 177, in load_and_cache_examples
    return HG_Datasets(tokenizer, file_path, args)
  File "/scratch/chiyuzh/roberta/run_language_modeling_hg.py", line 117, in HG_Datasets
    dataset = load_dataset('/scratch/chiyuzh/roberta/text.py', data_files=files, cache_dir = args.data_cache_dir, split="train")
  File "/arc/project/chiyuzh/evn_py36/datasets/src/datasets/load.py", line 590, in load_dataset
    path, script_version=script_version, download_config=download_config, download_mode=download_mode, dataset=True
  File "/arc/project/chiyuzh/evn_py36/datasets/src/datasets/load.py", line 385, in prepare_module
    os.makedirs(hash_folder_path)
  File "/project/chiyuzh/evn_py36/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
OSError: [Errno 30] Read-only file system: '/home/chiyuzh/.cache/huggingface/modules/datasets_modules/datasets/text/512f465342e4f4cd07a8791428a629c043bb89d55ad7817cbf7fcc649178b014'

```

I installed datasets at /project/chiyuzh/evn_py36/datasets/src where is a writable directory.
I also tried change the environment variables to the writable directory:
`export HF_MODULES_PATH=/project/chiyuzh/evn_py36/datasets/cache_dir/`
`export HF_DATASETS_CACHE=/project/chiyuzh/evn_py36/datasets/cache_dir/`
 
In my scripts, I also changed to:
`dataset = load_dataset('/scratch/chiyuzh/roberta/text.py', data_files=files, cache_dir = args.data_cache_dir, split="train")`
`data_cache_dir = $TMPDIR/data/` that also a writable directory.
 
But it still try to make directory at /home/chiyuzh/.cache/huggingface/modules/.
Do you have any idea about this issue? @thomwolf