Add DETA
# What does this PR do?

This PR adds [DETA](https://github.com/jozhang97/DETA/issues/3). DETA is a slight change to Deformable DETR by using traditional IoU-based assignment as opposed to the Hungarian matching used in the original DETR, and incorporating NMS (non-maximum suppression) in the postprocessing.

Note: this model has a `torchvision` dependency for NMS.

To do:

- [x] transfer checkpoints
cc @alaradirik this PR is in a ready state, except for 2 things:

- [x] whether or not we leverage torchvision's `batched_nms` => the CI is currently failing because this library is not installed. Will also ask for @sgugger and @LysandreJik's opinion here
- [ ] the `post_process_object_detection` method might require an in-depth look<|||||>There is no problem with the model requiring torchvision to be installed. We have many models with specific dependencies, some of which you ported yourself ;-).
Just protect the import between `if is_torchvision_available()` and have a the first line in the init of the models be a `require_backends(["torchvision"])`.<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger I've addressed all comments, except for adding support for the custom kernel.

Could we perhaps add support for the custom kernel for the 3 models (Mask2Former, OneFormer and DETA) in a separate PR?<|||||>In this case, remove the code trying to load the custom kernels in the modeling file and we can add it back in the PR that will deal with custom kernels.<|||||>@sgugger ok, feel free to approve :)<|||||>Failing test is unrelated/flaky, merging.