Adding CVT Model
# What does this PR do?
Add CvT Model for Vision Classification

Fixes #13158


## 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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      Pull Request section?
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      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] 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. Feel free to tag
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@NielsRogge @LysandreJik 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@NielsRogge can you review it and suggest any further changes.
I'm not sure how you want to modify `cls_token` part. Since modifying in `CvtStage` section (stopping the split) will change shape of hidden states (stored in all hidden states and passing of 4D shape for CNN in next layer).

I leave that part to you on how you want to change it to pass it over different classes further.

I have run the `make fix-copies` and done docstrings part. I think everything is done apart from change you wanted to make for `cls_token`.

