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 ,

Thanks for your suggestion. 
I tried to process the dataset and save it to disk. 
I have 1.12B samples in the raw dataset. I used 16 processors.
I run this process job for 7 days. But it didn't finish. I don't why the processing is such slow. 

The log shows that some processors (\#12, \#14, \#15) are very slow. The different processor has a different speed. These slow processors look like a bottleneck. 

Could you please give me any suggestion to improve the processing speed? 

Thanks. 
Chiyu

Here is my code:
```
def token_encode(examples):
        tokenizer_out = tokenizer(examples['text'], truncation=True,  padding="max_length", add_special_tokens=True, max_length=args.block_size)
        return tokenizer_out

path = Path(file_path)
files = sorted(path.glob('*'))
dataset = load_dataset('./text.py', data_files=files, cache_dir = args.data_cache_dir, split="train")
dataset = dataset.map(token_encode, batched=True, batch_size = 16384, num_proc = 16)
dataset.set_format(type='torch', columns=['input_ids', 'attention_mask'])
dataset.save_to_disk(output_dir)
```
Here is the log. 
```
^M#6:   1%|▏         | 59/4288 [55:10<66:11:58, 56.35s/ba]
^M#1:   8%|▊         | 356/4288 [55:39<10:40:02,  9.77s/ba]
^M#2:   5%|▍         | 210/4288 [55:33<17:47:19, 15.70s/ba]
^M#0:  19%|█▉        | 836/4288 [55:53<4:08:56,  4.33s/ba]
^M#0:  20%|█▉        | 837/4288 [55:57<4:01:52,  4.21s/ba]
^M#1:   8%|▊         | 357/4288 [55:48<10:38:09,  9.74s/ba]
^M#0:  20%|█▉        | 838/4288 [56:01<4:02:56,  4.23s/ba]
^M#3:   4%|▎         | 155/4288 [55:43<24:41:20, 21.51s/ba]
^M#0:  20%|█▉        | 839/4288 [56:05<4:04:48,  4.26s/ba]
^M#12:   1%|          | 29/4288 [54:50<133:20:53, 112.72s/ba]
^M#2:   5%|▍         | 211/4288 [55:48<17:40:33, 15.61s/ba]
^M#14:   0%|          | 2/4288 [04:24<157:17:50, 132.12s/ba]
^M#15:   0%|          | 1/4288 [02:24<172:11:37, 144.60s/ba]
```