Add OWL-ViT model for zero-shot object detection
# What does this PR do?
- Adds OwlViT model for open-vocabulary object detection. Model takes in one or multiple text queries per image as input.

Original repo:
https://github.com/google-research/scenic/tree/a41d24676f64a2158bfcd7cb79b0a87673aa875b/scenic/projects/owl_vit

Test notebook:
https://colab.research.google.com/drive/1IMPWZcnlMy-tdnTDrUcOZU3oiGg-hTem?usp=sharing

@sgugger could you review my draft PR, please?

_The documentation is not available anymore as the PR was closed or merged._<|||||>Any plan to extend it for TensorFlow version? 
There seems to be [conversion script](https://github.com/google-research/scenic/tree/a41d24676f64a2158bfcd7cb79b0a87673aa875b/scenic/projects/owl_vit#conversion-to-tensorflow) officially. <|||||>Hi @innat. Yes, @alaradirik is already working on it! The PR is here: https://github.com/huggingface/transformers/pull/18450

You can find out which models are being implemented by searching the open issues and PRs [for example](https://github.com/huggingface/transformers/pulls?q=is%3Apr+is%3Aopen+owlvit)