BertForSequenceClassification() model TF to pytorch conversion
I added a script convert_bert_seqclass_tf_checkpoint_to_pytorch.py for the conversion a trained BertForSequenceClassification model from TF to pytorch.
I had to modify modeling_bert.py to support it, as well.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2170?src=pr&el=h1) Report
> Merging [#2170](https://codecov.io/gh/huggingface/transformers/pull/2170?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c8ed1c82c8a42ef700d4129d227fa356385c1d60?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2170/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2170?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2170      +/-   ##
==========================================
- Coverage   80.35%   80.34%   -0.01%     
==========================================
  Files         114      114              
  Lines       17095    17097       +2     
==========================================
  Hits        13736    13736              
- Misses       3359     3361       +2
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2170?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2170/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2JlcnQucHk=) | `87.43% <0%> (-0.32%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2170?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/transformers/pull/2170?src=pr&el=footer). Last update [c8ed1c8...e3c65da](https://codecov.io/gh/huggingface/transformers/pull/2170?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
