Add type hints OpenAIGPT
This PR adds type annotations for both pytorch and tensorflow implementations of OpenAIGPT model.
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Hi @TristanBilot, I'm sorry I didn't see this PR until now! It looks like those models have had type hints added by other PRs. Also, this PR is quite hard to review because of the rebase, which has made the list of "files changed" really large. Can you check the `OpenAIGPT` model files to see if we still need a PR for this, and if we do, can you make a fresh one based on the latest version of `main`?<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.