changes in evaluate function in run_lm_finetuning.py
changed the return value of `evaluate` function from `results` to `result` and also removed unused empty dict `results`
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1229?src=pr&el=h1) Report
> Merging [#1229](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1229?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/84d346b68707f3c43903b122baae76ae022ef420?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1229/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1229?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1229   +/-   ##
=======================================
  Coverage   81.22%   81.22%           
=======================================
  Files          57       57           
  Lines        8027     8027           
=======================================
  Hits         6520     6520           
  Misses       1507     1507
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1229?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1229?src=pr&el=footer). Last update [84d346b...4b082bd](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1229?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks!