Add DETR
# What does this PR do?

It adds Facebook AI's DETR model (end-to-end object detection with Transformers). It's a clean PR based on #11506. 

The 3 models are called `DetrModel`, `DetrForObjectDetection` and `DetrForSegmentation`. The latter was first called `DetrForPanopticSegmentation`, but as it can also be used to do only instance segmentation, I renamed it. 

To do:

- [x] address remaining comments
- [x] fix remaining tests (there are still 2 tests failing for `test_modeling_detr.py`) - here I'd like some help
- [x] add remaining checkpoints to the hub 
- [ ] add notebooks to showcase how to do inference/fine-tuning on custom data
- [x] perhaps also write more documentation
@LysandreJik all comments are addressed, also added 2 community notebooks. PR is ready!