Add MSN checkpoints to ViT
### Feature request

It's been a while Meta released the [MSN](https://github.com/facebookresearch/msn) (Masked Siamese Networks) checkpoints. MSN is known to perform pretty well in few-shot settings. Few-shot regime is important for real-world applications. They have only released the encoder weights.

I have worked on a [Colab Notebook](https://colab.research.google.com/gist/sayakpaul/51556ce57facea02041cf0c0f1c3fed3/scratchpad.ipynb) that converts the released MSN checkpoints into HF format and performs assertions to ensure the converted weights can be loaded into HF's ViT classes.

This issue proposes adding the MSN checkpoints to be compatible with HF's ViT classes. 

### Motivation

MSN is particularly useful in few-shot learning regimes.

### Your contribution

Can contribute to the checkpoints and other related utilities.

@NielsRogge could you assign this issue to me?