Fix for mlm evaluation in run_lm_finetuning.py
No masking is done in the original evaluation code, so the resulting perplexity is always something like 1.0. In this PR a simple fix is proposed, using just the same masked scheme as in a training code.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1633?src=pr&el=h1) Report
> Merging [#1633](https://codecov.io/gh/huggingface/transformers/pull/1633?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ae1d03fc51bb22ed59517ee6f92c560417fdb049?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/1633/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1633?src=pr&el=tree)

```diff
@@          Coverage Diff           @@
##           master   #1633   +/-   ##
======================================
  Coverage    85.9%   85.9%           
======================================
  Files          91      91           
  Lines       13653   13653           
======================================
  Hits        11728   11728           
  Misses       1925    1925
```



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<|||||>This is great, thanks!