Add BiT + ViT hybrid
# What does this PR do?

This PR adds ViT hybrid to the library. As ViT hybrid uses BiT as backbone, this PR also adds BiT as a standalone model.

BiT itself is very similar to a ResNetv2, except that it replaces batch norm layers by group norm and uses "weight standardized" convolutional layers.

To do:

- [x] add image processors
- [ ] add tests for image processors (cc @amyeroberts can I directly add test_modeling_image_processor_xxx.py ?)
- [ ] transfer all checkpoints
- [x] add integration tests
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks so much @sgugger for your review ! 
I should have updated everything and the main models are now up:
- https://huggingface.co/google/vit-hybrid-base-bit-384 
- https://huggingface.co/google/bit-50