Fast tokenizers don't properly tokenize special tokens
Slow tokenizers:

```
>>> import transformers
>>> t_slow = transformers.AutoTokenizer.from_pretrained("roberta-base", use_fast=False)
>>> t_slow.encode_plus("A <mask> sentence.")
{'input_ids': [0, 83, 50264, 3645, 4, 2],
 'token_type_ids': [0, 0, 0, 0, 0, 0],
 'attention_mask': [1, 1, 1, 1, 1, 1]}

>>> t_slow.convert_ids_to_tokens([0, 83, 50264, 3645, 4, 2])
['<s>', 'ĠA', '<mask>', 'Ġsentence', '.', '</s>']
```

Fast tokenizers:
```
>>> import transformers
>>> t_fast = transformers.AutoTokenizer.from_pretrained("roberta-base", use_fast=True)
>>> t_fast.encode_plus("A <mask> sentence.")
{'input_ids': [0, 250, 1437, 50264, 3645, 4, 2],
 'token_type_ids': [0, 0, 0, 0, 0, 0, 0],
 'attention_mask': [1, 1, 1, 1, 1, 1, 1]}

>>> t_fast.convert_ids_to_tokens([0, 250, 1437, 50264, 3645, 4, 2])
['<s>', 'A', 'Ġ', '<mask>', 'Ġsentence', '.', '</s>'] 
```
I think it's a duplicate of #2919. Btw, I should have told you when I saw you open the PR, [I opened a bunch of issues related to AllenNLP usage](https://github.com/huggingface/transformers/issues?utf8=%E2%9C%93&q=is%3Aissue+author%3Abryant1410). I think one that was closed it's not completely solved, but not sure.<|||||>It's not the same issue. This one is about special tokens in text form in the middle of your string. #2919 is about `[CLS]` and `[SEP]` being added to the beginning and end. Also, #2919 has already been fixed.<|||||>Ohh, :+1: <|||||>This is now fixed on `master`:

```
>>> import transformers
>>> t_fast = transformers.AutoTokenizer.from_pretrained("roberta-base", use_fast=True)
>>> t_fast.encode_plus("A <mask> sentence.")
{'input_ids': [0, 83, 50264, 3645, 4, 2], 'attention_mask': [1, 1, 1, 1, 1, 1]}
>>> t_fast.convert_ids_to_tokens([0, 83, 50264, 3645, 4, 2])
['<s>', 'ĠA', '<mask>', 'Ġsentence', '.', '</s>']
```