chore: add py.typed in compliance with PEP 561

[niqodea]

What does this PR do?

Analogously to what has been done in this PR for diffusers, do the same for transformers.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
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• Did you make sure to update the documentation with your changes? Here are the
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  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed.

[amyeroberts]

Hi @niqodea, thanks for opening this PR!

Transformers doesn't officially support type checking in the library. Type annotations are used as documentation but we don't guarantee that they are consistent and checked across the repo. See: #18485

[niqodea]

Ah, just noticed that there has already been an attempt to integrate py.typed previously (link). However, I currently don't see much complaining in my project as of now. Maybe we can try integrating it again? Feel free to close this otherwise.

[niqodea]

@amyeroberts yep, makes sense. Thanks for your review!