[DETR and friends] Remove is_timm_available
# What does this PR do?

This PR:
- removes the `is_timm_available` dependency check for DETR and friends, uses `is_torch_available` instead and uses `requires_backends["timm"]` in case `config.use_timm_backbone=True`.
- adapts DETR's conversion script to make DETR work with our `AutoBackbone` class, rather than the timm backbone. This way one can use DETR by only installing `Transformers`. 

To do:

- [x] upload checkpoint to the hub
- [x] add integration test
_The documentation is not available anymore as the PR was closed or merged._<|||||>There is a pretty big conflict in the test modeling DETR file, can you fix it Niels?