Add DINO conversion script
# What does this PR do?

I've uploaded the Vision Transformers trained using the self-supervised method called [DINO](https://github.com/facebookresearch/dino) to the hub: https://huggingface.co/models?other=dino

This PR includes the conversion script that was used.

I've also added a reference to DeiT, BEiT and the DINO checkpoints to the docs of ViT, to give these models some more love.

