Add type hints to xlnet
Author: mowafess
Pull request: yes

# What does this PR do?

I added type annotations for xlnet (PT and TF) as described in [#16059]

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## Before submitting
- [ ] 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?


## Who can review?
@Rocketknight1 @patrickvonplaten @sgugger

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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Models:

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- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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- text generation: @patrickvonplaten
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HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
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Comments:
_The documentation is not available anymore as the PR was closed or merged._