SwinModel docstring fix

[shivanandmn]

What does this PR do?

Docstring for SwinModel is added.

Fixes # (issue)

Before submitting

• [x ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ x] Did you read the contributor guideline,
  Pull Request section?
• [x ] Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case. [Community Event] Docstring Sprint #26638
• [ x] Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@ydshieh

[shivanandmn]

@ydshieh resolved merge conflicts - ready for ✅ Thanks!

[ydshieh]

Ready to go once suggestion applied 🙏

Thank you for the contribution 🚀

[shivanandmn]

@ydshieh
I have updated changes. This is my first open-source contribution in the life.
I happy to learn from community.
Thanks

[ydshieh]

Thank you for your (first) contribution 🥳 🎉 !

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.