GPT2 TensorFlow Type Hints
# What does this PR do?
Add type hints to the TF GPT2 model

Also remove two unused imports that were breaking `make fixup`

<!-- Remove if not applicable -->
Addresses  https://github.com/huggingface/transformers/issues/16059


## 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.
- [ ] Did you make sure to update the documentation with your changes? 
- [ ] Did you write any new necessary tests?


## Who can review?

@Rocketknight1 @gante 

Potential issues (not very sure about those): 
- Pytorch GPT2 has a `GPT2ForTokenClassification` class with no equivalent in the TF version.
-  TF GPT2 uses `past` for the parameter that other models (e.g. pytorch gpt2 and tf roberta) call `past_key_values`<|||||>_The documentation is not available anymore as the PR was closed or merged._