[DETR and friends] Use AutoBackbone as alternative to timm
# What does this PR do?

This PR makes it possible to leverage our own backbone classes, like ResNet or Swin Transformer, instead of relying on timm for the following models:
- DETR
- Conditional DETR
- Deformable DETR
- Table Transformer

 This allows people to use these frameworks without having to rely on the timm dependency.

I've added an attribute to the config "use_timm_backbone" which is set to True by default, but can be set to False.

To do:

- [x] fix copies, once design gets approved
_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.<|||||>@sgugger feel free to approve :)<|||||>Can you make all tests pass before asking for a final review?<|||||>The remaining tests which are failing are due to `make fix-copies`, however I'll only start updating the other models once this design is approved.<|||||>@sgugger thanks for the review, addressed all comments!