Added TF OpenAi GPT1 Sequence Classification
This PR implements Sequence classification for TF OpenAi GPT1 model.
TFOpenAIGPTForSequenceClassification uses the last token in order to do the classification, as other causal models (e.g. Transformer XL ,GPT-2) do.

Fixes #7623 


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?
@LysandreJik @jplu 

Not sure, why test cases are failing for 'run_tests_tf'. @LysandreJik . 
Let me know any action require from my side.<|||||>Can you rebase on master, a fix has been recently merged.<|||||>The tests have already been fixed on `master`, merging! Thanks a lot @spatil6