Improve DETR models
# What does this PR do?

This PR: 

- [x] fixes Deformable DETR's loss function
- [x] adds more copied from statements for consistency
- [x] fixes Conditional DETR's integration tests.

As pointed out in #18948, Deformable DETR uses the same loss function and Hungarian matcher as Conditional DETR (use of sigmoid instead of softmax and not including the no-object class).

This PR also improves the  (original; conditional; deformable) DETR models by improving docs, adding more Copied from statements.

_The documentation is not available anymore as the PR was closed or merged._