RobertaTokenizerFast unexpectedly quits when creating a TextDataset
# 🐛 Bug

When creating a `TextDataset` using `RobertaTokenizerFast` my program unexpectedly dies. (Not so with `RobertaTokenizer`).

## Information

Model I am using: RoBERTa

Language I am using the model on: English

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [X] my own modified scripts: (give details below)

The tasks I am working on is:
* [X] an official GLUE/SQUaD task: [language modelling](https://github.com/huggingface/transformers/blob/33d3072e1c54bcd235447b98c6dea1b4cb71234c/examples/run_lm_finetuning.py)
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

```
from transformers import AutoTokenizer, TextDataset

tokenizer = AutoTokenizer.from_pretrained("roberta-base", use_fast=True)

train_dataset = TextDataset(
    tokenizer=tokenizer,
    file_path="/home/ubuntu/data/wikitext-103-raw/wiki.train.raw",
    block_size=-1,
    overwrite_cache=False,
)
print(train_dataset)
```
## Expected behavior

Creation of the training dataset, not having the process killed. eg:

```
<transformers.data.datasets.language_modeling.TextDataset object at 0x7f138a1fd2b0>
```

## Environment info

- `transformers` version: 2.11.0
- Platform: Linux-5.3.0-1030-aws-x86_64-with-debian-buster-sid
- Python version: 3.7.7
- PyTorch version (GPU?): 1.5.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No


@n1t0 <|||||>This seems to work for me, I guess it crashes because you don't have enough memory. Unfortunately `TextDataset` has not been optimized for fast tokenizers yet, so it does a lot more work than needed when using them. It's probably better to use python tokenizers for now with `TextDataset`.

Also, maybe the [huggingface/nlp](https://github.com/huggingface/nlp) library might be better suited here. cc @lhoestq <|||||>You could try
```python
from transformers import AutoTokenizer
from nlp import load_dataset

tokenizer = AutoTokenizer.from_pretrained("roberta-base", use_fast=True)
dataset = load_dataset("text", data_files="path/to/wiki.train.raw", split="train")
tokenized_dataset = dataset.map(lambda ex: tokenizer(ex["text"]), batched=True)
print(tokenized_dataset[0]["input_ids"])
```
We're still working on making it as fast as we can, but at least you won't have any memory issues.<|||||>Re @n1t0 comment: "I guess it crashes because you don't have enough memory" this is correct. (I was hoping I could get away with 61.0 GiB, the standard for an AWS `p3.2xlarge`.)

Re @lhoestq your code ran without errors for me. Thanks! 

I did get a lot of the [`Token indices sequence length is longer than the specified maximum sequence length for this model (522 > 512). Running this sequence through the model will result in indexing errors`](https://github.com/huggingface/transformers/issues/1791) warnings which I wasn't getting before.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
