add vit tf doctest with @add_code_sample_docstrings

[johko]

What does this PR do?

Creates doctest for TF version of ViT. It is a cleaner version of PR #16462 which had a messed up history.

Fixes # (issue)

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
  to it if that's the case.
• 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

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[ydshieh]

Thanks a lot @johko!

Very clean PR 💯 . One minor thing to fix, and we are ready to merge it.

[ydshieh]

LGTM, thank you @johko !

I ran it locally and the the doctests passed!

@sgugger Could you have a final look too?

[sgugger]

LGTM, thanks a lot for your PR!