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 
 Hi @lhoestq ,

I tried to use multi-processor, but I got errors as follow: 
Because I am using python distributed training, it seems some conflicts with the distributed job. 

Do you have any suggestions?
```
I0925 10:19:35.603023 140737353971520 filelock.py:318] Lock 140737229443368 released on /tmp/pbs.1120510.pbsha.ib.sockeye/cache/_tmp_pbs.1120510.pbsha.ib.sockeye_cache_text_default-7fb934ed6fac5d01_0.0.0_512f465342e4f4cd07a8791428a629c043bb89d55ad7817cbf7
fcc649178b014.lock
Traceback (most recent call last):
  File "/scratch/chiyuzh/roberta/run_language_modeling.py", line 1024, in <module>
    main()
  File "/scratch/chiyuzh/roberta/run_language_modeling.py", line 967, in main
    train_dataset = load_and_cache_examples(args, tokenizer, evaluate=False)
  File "/scratch/chiyuzh/roberta/run_language_modeling.py", line 180, in load_and_cache_examples
    return HG_Datasets(tokenizer, file_path, args)
  File "/scratch/chiyuzh/roberta/run_language_modeling.py", line 119, in HG_Datasets
    dataset = dataset.map(token_encode, batched=True, batch_size = 10000, num_proc = 16)
  File "/project/chiyuzh/evn_py36/lib/python3.6/site-packages/datasets/arrow_dataset.py", line 1287, in map
    transformed_shards = [r.get() for r in results]
  File "/project/chiyuzh/evn_py36/lib/python3.6/site-packages/datasets/arrow_dataset.py", line 1287, in <listcomp>
    transformed_shards = [r.get() for r in results]
  File "/project/chiyuzh/evn_py36/lib/python3.6/multiprocessing/pool.py", line 644, in get
    raise self._value
  File "/project/chiyuzh/evn_py36/lib/python3.6/multiprocessing/pool.py", line 424, in _handle_tasks
    put(task)
  File "/project/chiyuzh/evn_py36/lib/python3.6/multiprocessing/connection.py", line 206, in send
    self._send_bytes(_ForkingPickler.dumps(obj))
  File "/project/chiyuzh/evn_py36/lib/python3.6/multiprocessing/reduction.py", line 51, in dumps
    cls(buf, protocol).dump(obj)
AttributeError: Can't pickle local object 'HG_Datasets.<locals>.token_encode'
```