Add TFGPT2ForSequenceClassification based on DialogRPT
# What does this PR do?

This PR implements TFGPT2ForSequenceClassification in order to support DialogRPT.
Strongly based on modifications made in #7501
Fixes #7622 


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
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      [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?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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@LysandreJik Please review this PR, let me know if there is anything that should be changed =)
Thanks for review @jplu . I'll update my code with review comments and new input processing.
<|||||>> Thank you very much for this very nice addition!!
> 
> I left few comments on it. Also can you run the following piece of code and tell me if it works properly:
> 
> ```
> import tensorflow as tf
> from transformers import GPT2Tokenizer, TFGPT2ForSequenceClassification
> 
> model = tf.function(TFGPT2ForSequenceClassification.from_pretrained("microsoft/dialogrpt"))
> tokenizer = GPT2Tokenizer.from_pretrained("microsoft/dialogrpt")
> inputs = tokenizer("Hello", return_tensors="tf")
> model(inputs)
> ```
> 
> @LysandreJik I would recommend as well to wait a bit that the new input processing to be merged.

<img width="1176" alt="output" src="https://user-images.githubusercontent.com/6419011/100481765-2c2da900-311b-11eb-8fdd-15762f7d43df.png">
<|||||>Hello @jplu and @LysandreJik ,
I have refactored code as per review comments and added new input processing as well.

Kindly review.<|||||>> Much better!! Thanks for the updates.
> 
> There is still one comment to be addressed and the tests to fix.

@jplu tests are also fixed now.<|||||>@spatil6 we have merged today a PR that updates the way the booleans are processed. You can see an example in the TF BERT file for example, can you rebase and proceed to the same changes please. It would be awesome if you could do it!<|||||>> @spatil6 we have merged today a PR that updates the way the booleans are processed. You can see an example in the TF BERT file for example, can you rebase and proceed to the same changes please. It would be awesome if you could do it!

Sure, will do that.