Adding LeViT Model by Facebook
Adds LeViT Model (a Vision Transformer) to HF Library. All the checkpoints are on my HF account hub.
Please review and suggest the required changes.
@NielsRogge 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for suggestion. I have done the requested changes. I need help here:
- I cannot figure rendering of warning problem. If I give `enter` or `space`. There is `style error` of `white space`.
- The image in `convnext.mdx` is added in HF Dataset repo in hub. I can give the image to add there.

The weights are also uploaded on hub correctly after change in `self.bn` -> `self.batch_norm`.
Please review and suggest the changes @NielsRogge <|||||>CI error seems unrelated, thanks for your work, merging!