Vit deit fixes
# What does this PR do?

* Some small documentation improvements of ViT + DeiT.
* Adds a cats image to the `fixtures/test_samples` folder, which is used in the integration tests of both ViT and DeiT. 
* Adds a community notebook, illustrating how to fine-tune the Vision Transformer on CIFAR-10.

(there's something weird going on with the .gitignore within the test_samples folder , see files changed). 
The notebook looks amazing!

Out of curiosity, have you tried using `Trainer` to fine-tune `ViT`?<|||||>Added a notebook that uses the Trainer. Includes a nice confusion matrix at the end :) 