Owlvit docs test
- Fixes a typo in OwlViTForObjectDetection forward function docs: transformers/models/owlvit/modeling_owlvit.py
- Adds docs test for OWL-ViT
- Makes `OwlViTFeatureExtractor.post_process` callable from `OwlViTProcessor`
- Improves code examples to demonstrate how to use the post_process method
_The documentation is not available anymore as the PR was closed or merged._<|||||>> LGTM! Just wondering why there are 90+ commits

That was my mistake, I merged the owlvit branch of my forked transformers repo with the main and created this branch. I squashed the commits on the main but don't know how to fix this one.<|||||>Let me know if you'd like some help to squash the commits of this PR @alaradirik!