Fix TF optimization classes and apply the changes in the NER TF script
This PR fix the AdamW and GradientAccumulator optimization classes for Tensorflow. Furthermore, we apply these new changes in the NER TF script.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3907?src=pr&el=h1) Report
> Merging [#3907](https://codecov.io/gh/huggingface/transformers/pull/3907?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/cb3c2212c79d7ff0a4a4e84c3db48371ecc1c15d&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3907/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3907?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3907   +/-   ##
=======================================
  Coverage   78.45%   78.45%           
=======================================
  Files         111      111           
  Lines       18521    18521           
=======================================
  Hits        14531    14531           
  Misses       3990     3990           
```



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