fixes issue with unrecognized arguments for AdamW
as suggested in https://github.com/huggingface/transformers/issues/830
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1891?src=pr&el=h1) Report
> Merging [#1891](https://codecov.io/gh/huggingface/transformers/pull/1891?src=pr&el=desc) into [xlnet](https://codecov.io/gh/huggingface/transformers/commit/1b35d05d4b3c121a9740544aa6f884f1039780b1?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/1891/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1891?src=pr&el=tree)

```diff
@@          Coverage Diff          @@
##           xlnet   #1891   +/-   ##
=====================================
  Coverage   78.9%   78.9%           
=====================================
  Files         34      34           
  Lines       6181    6181           
=====================================
  Hits        4877    4877           
  Misses      1304    1304
```



------

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<|||||>Same as #1890 