Add FocalNet
# What does this PR do?

This PR adds FocalNet to the library.

To do:

- [x] remove print_values
- [x] Upload checkpoints
- [x] Update model cards
- [x] Update integration tests
- [x] Transfer checkpoints
_The documentation is not available anymore as the PR was closed or merged._<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>CC @NielsRogge 

I cleaned up the config dependent arguments a bit, uploaded the models and edited the model cards. @amyeroberts @sgugger could you take another look?

FocalNet is the backbone of X-Decoder so it'd be great it to merge the PR soon.