Add general vision docstrings
# What does this PR do?

This is an attempt to add general docstrings for vision models, specifically for the base model and the image classification ones.

To do:

- [x] ViT
- [x] DeiT
- [x] BEiT
- [x] SegFormer
- [x] Swin Transformer
_The documentation is not available anymore as the PR was closed or merged._