Add GIT (GenerativeImage2Text)
# What does this PR do?

This PR implements GIT, short for GenerativeImage2Text. The model is a decoder-only Transformer conditioned on CLIP patch tokens + text tokens for tasks like image captioning and VQA.

To do:

- [x] add support for user-provided attention_mask
- [x] fix model_input_names of processor, see #20549 
- [x] fix issue where GitProcessor seems to instantiate a `VideoMAEImageProcessor` by default
- [x] transfer checkpoints to `microsoft`
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger PR is ready for a final review :) only remaining issue is that we need to update `from_pretrained` in `ProcessorMixin` to handle multiple image processors<|||||>Hi, is it possible to finetune git with transformers?