make RobertaForMaskedLM implementation identical to fairseq
closes https://github.com/huggingface/transformers/issues/1874

The implementation of RoBERTa in `transformers` differs from the original implementation in [fairseq](https://github.com/pytorch/fairseq/tree/master/fairseq/models/roberta), as results showed (cf. https://github.com/huggingface/transformers/issues/1874). I have documented my findings here https://github.com/huggingface/transformers/issues/1874#issuecomment-588359143 and made the corresponding changes accordingly in this PR.

Someone should check, however, that removing `get_output_embeddings()` does not have any adverse side-effects.
Awesome. Could you check if the existing @slow tests break for Roberta, and add a new one that hardcodes the fairseq logits from your example and makes sure we also return them. Trying to avoid accidental breakage. Thanks again! <|||||>@sshleifer Not sure how I would hardcode a tensor of size 1, 12, 50265. Can I just add a small pickled file to the test instead?<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2913?src=pr&el=h1) Report
> Merging [#2913](https://codecov.io/gh/huggingface/transformers/pull/2913?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/59c23ad9c931ac4fe719abeb3c3851df046ef3a6?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `85.71%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2913/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2913?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2913      +/-   ##
==========================================
- Coverage    75.3%   75.29%   -0.01%     
==========================================
  Files          94       94              
  Lines       15424    15424              
==========================================
- Hits        11615    11614       -1     
- Misses       3809     3810       +1
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2913?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2913/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `95.3% <85.71%> (-0.47%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2913?src=pr&el=footer). Last update [59c23ad...51514a4](https://codecov.io/gh/huggingface/transformers/pull/2913?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Sorry for the close, had to do some rebasing.