Update conversion script to convert XLM-R
It requires a few changes to the existing RoBERTa conversion script because `fairseq` has updated the RoBERTa model definition. 

Specifically, the definition of multiheaded attention is changed at https://github.com/pytorch/fairseq/blob/2a9b4ec2374574cd0315f95e126788e4fe795f0d/fairseq/modules/multihead_attention.py#L42

With this new script, you should be able to convert the XLM-R model released at https://github.com/pytorch/fairseq/tree/master/examples/xlmr and requested by #1769. 

I have successfully converted the two released model weights. 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1812?src=pr&el=h1) Report
> Merging [#1812](https://codecov.io/gh/huggingface/transformers/pull/1812?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/155c782a2ccd103cf63ad48a2becd7c76a7d2115?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1812/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1812?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1812   +/-   ##
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  Coverage   84.16%   84.16%           
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  Files          94       94           
  Lines       14185    14185           
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  Hits        11939    11939           
  Misses       2246     2246
```



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<|||||>This is great indeed but we are losing backward compatibility to convert the original RoBERTa weights aren't we?

Maybe we can add a flag to control which version of fairseq RoBERTa the original weight come from? For instance a flag as a string selected between `roberta` and `XLM-R`.<|||||>I think the situation is a bit more complicated than I thought. We also need to change the vocabulary size, etc. I think it would be better to have a separate treatment for XLM-R. So I am closing this for now. 