Add type hints for BERTGeneration
# What does this PR do?
I added type hints for the `BERTGenerationEncoder` and `BERTGenerationDecoder` classes as requested in [#16059](https://github.com/huggingface/transformers/issues/16059) and demonstrated in [#16074](https://github.com/huggingface/transformers/pull/16074). 

I wasn't completely sure on what to use for the `past_key_values` argument. I set it to `Optional[Tuple[Tuple[torch.FloatTensor]]]`, but let me know if this is wrong. Also, not sure if I should also add type hints for the `BertGenerationConfig` class?

@Rocketknight1 
_The documentation is not available anymore as the PR was closed or merged._<|||||>> I wasn't completely sure on what to use for the `past_key_values` argument. I set it to `Optional[Tuple[Tuple[torch.FloatTensor]]]`, but let me know if this is wrong. Also, not sure if I should also add type hints for the `BertGenerationConfig` class?

What you put here is fine, and type hints for `BertGenerationConfig` are nice but optional - if you want to do them you can, but the main thing we're interested in is the core model classes. Let me know either way - if you don't want to do it, this is ready to merge now!<|||||>Okay great, you can go ahead and merge it then. I'll run your notebook to see what else needs to be done and work on some of those instead. Cheers<|||||>Got it. Thanks for the PR!