XLMRobertaTokenizer vocab size
I think the XLMRobertaTokenizer vocab_size is off. Currently double counts ```'<unk>' | '<s>' | '</s>'```

Maybe change it to
```
def vocab_size(self):
    return len(self.sp_model) + self.fairseq_offset
```

Running the following code caused error for me: 

> import transformers
> tokenizer = transformers.AutoTokenizer.from_pretrained("xlm-roberta-base")
> tokenizer.convert_ids_to_tokens(range(tokenizer.vocab_size))

Actually, the `tokenizer.vocab_size` is `250005`, the last id `250004` is `<mask>`, but the ids from `250001` to `250003` do not exist.<|||||>> Actually, the `tokenizer.vocab_size` is `250005`, the last id `250004` is `<mask>`, but the ids from `250001` to `250003` do not exist.

Ya ok this is definitely the problem. Either way, it's an issue for the current implementation of get_vocab which will crash at 25001:

```
    def get_vocab(self):
        vocab = {self.convert_ids_to_tokens(i): i for i in range(self.vocab_size)}
        vocab.update(self.added_tokens_encoder)
        return vocab
```
<|||||>I wonder if this issue will be fixed? Currently it is not...<|||||>This issue is known and will be fixed.<|||||>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.
<|||||>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.
<|||||>This should have been fixed with https://github.com/huggingface/transformers/pull/3198