Add type hints for GPTNeo PyTorch
Adding type hints for forward methods in user-facing class for GPTNeo (PyTorch) as mentioned in #16059  

@Rocketknight1 
_The documentation is not available anymore as the PR was closed or merged._<|||||>If you run `make style` from the root of the repo and push again then it should fix the failing the tests.<|||||>@patil-suraj Thanks for handling this one, feel free to merge it whenever you're happy!<|||||>@Tegzes if you change the type for `past_key_values` from `List` to `Tuple` we should be good to merge :) <|||||>Awesome, I'm making the changes right now