Type hints MCTCT
Hi @Rocketknight1: this PR looks to add type hints to MCTCT models and addresses #16059.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [ ] Did you write any new necessary tests?
_The documentation is not available anymore as the PR was closed or merged._<|||||>Ah, careful! `attention_mask` and `head_mask` **were** optional in the main model methods, but not in the encoder. The encoder mostly doesn't have `Optional` arguments, but the other classes do. The changes to the main model methods is why you're getting that error now!

This is much easier to see if you look at the 'files changed' interface on GitHub - we want this PR to only add type annotations, but not change any default arguments.<|||||>I see - my apologies! Will make some changes now to fix this :) 