fix xlm roberta tokenizer mask id
As per issue #2508 the xlm_roberta_tokenizer has an error in the mask_id computation.
The sp_model already contains all the special tokens (bos, pad, eos, unk) but the mask id, which, according to the model specification should be 250001 instead of 250004:
```
self.fairseq_tokens_to_ids["<mask>"] = len(self.sp_model) + self.fairseq_offset
```
instead of 
```
self.fairseq_tokens_to_ids["<mask>"] = len(self.sp_model) + len(self.fairseq_tokens_to_ids)
```
Hey @stefan-it, mind taking a look at this?<|||||>Hi,
I get the following error when trying to adapt the same code as BERT for Masked LM with XLM-Roberta For Masked LM, where I've replaced **'[MASK]'** with **'\<mask>'** and '**[CLS]'** and **'[SEP]'** with **\<s>** and **\</s>** respectively.

```
/usr/local/lib/python3.6/dist-packages/torch/nn/functional.py in embedding(input, weight, padding_idx, max_norm, norm_type, scale_grad_by_freq, sparse)
   1482         # remove once script supports set_grad_enabled
   1483         _no_grad_embedding_renorm_(weight, input, max_norm, norm_type)
-> 1484     return torch.embedding(weight, input, padding_idx, scale_grad_by_freq, sparse)
   1485 
   1486 

RuntimeError: index out of range: Tried to access index 250004 out of table with 250001 rows. at /pytorch/aten/src/TH/generic/THTensorEvenMoreMath.cpp:418
```

I'm sorta new to this. Can I know whether this is related to the above discussed issue? and is much obliged to know how this error can be fixed. 