Adding type hints of TF:OpenAIGPT
Issue related: #16059

As the title suggests, this PR adds type hints to the `Tensorflow` `OpenAIGPT` model class.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hey @Mathews-Tom, thanks for working on providing type hints for both OpenAIGPT and CTRL! To get a review quicker, don't hesitate to ping @Rocketknight1 directly :)