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 
 Thanks, @thomwolf  and @sipah00 ,

I tried to implement your suggestions in my scripts. 
Now, I am facing some connection time-out error. I am using my local file, I have no idea why the module request s3 database.

The log is:
```
Traceback (most recent call last):
  File "/home/.local/lib/python3.6/site-packages/requests/adapters.py", line 449, in send
    raise err
  File "/home/.local/lib/python3.6/site-packages/urllib3/util/connection.py", line 74, in create_connection
    timeout=timeout
  File "/home/.local/lib/python3.6/site-packages/urllib3/connectionpool.py", line 720, in urlopen
    sock.connect(sa)
TimeoutError: [Errno 110] Connection timed out

Traceback (most recent call last):
  File "/home/.local/lib/python3.6/site-packages/urllib3/connectionpool.py", line 672, in urlopen
    method, url, error=e, _pool=self, _stacktrace=sys.exc_info()[2]
  File "/home/.local/lib/python3.6/site-packages/urllib3/util/retry.py", line 436, in increment
    chunked=chunked,
  File "/home/.local/lib/python3.6/site-packages/urllib3/connectionpool.py", line 376, in _make_request
    raise MaxRetryError(_pool, url, error or ResponseError(cause))
urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='s3.amazonaws.com', port=443): Max retries exceeded with url: /datasets.huggingface.co/datasets/datasets/text/text.py (Caused by NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection obj
ect at 0x7fff401e0e48>: Failed to establish a new connection: [Errno 110] Connection timed out',))

Traceback (most recent call last):
  File "/scratch/roberta_emohash/run_language_modeling.py", line 1019, in <module>
    main()
  File "/scratch/roberta_emohash/run_language_modeling.py", line 962, in main
    train_dataset = load_and_cache_examples(args, tokenizer, evaluate=False)
  File "/scratch/roberta_emohash/run_language_modeling.py", line 177, in load_and_cache_examples
    return HG_Datasets(tokenizer, file_path, args)
  File "/scratch/roberta_emohash/run_language_modeling.py", line 117, in HG_Datasets
    dataset = load_dataset('text', data_files=files, cache_dir = args.data_cache_dir, split="train")
  File "/arc/project/evn_py36/datasets/datasets/src/datasets/load.py", line 590, in load_dataset
    self._validate_conn(conn)
  File "/home/.local/lib/python3.6/site-packages/urllib3/connectionpool.py", line 994, in _validate_conn
    conn.connect()
  File "/home/.local/lib/python3.6/site-packages/urllib3/connection.py", line 300, in connect
    conn = self._new_conn()
  File "/home/.local/lib/python3.6/site-packages/urllib3/connection.py", line 169, in _new_conn
    self, "Failed to establish a new connection: %s" % e
urllib3.exceptions.NewConnectionError: <urllib3.connection.VerifiedHTTPSConnection object at 0x7fff401e0da0>: Failed to establish a new connection: [Errno 110] Connection timed out

``` 

Do you have any experience on this issue?