Rename detr targets to labels
# What does this PR do?

It fixes #12248. As the models accept "labels" as an argument, it's better to also use the term "labels" in the feature extractor instead of "target". 

Note that if this PR gets merged, I'll need to update my demo notebooks (rename "target" to "labels").

I also improved the documentation a little more, and removed some unused variables from `DetrConfig`.

cc @LysandreJik 
@LysandreJik if you want, I can also update the failing integration test for `DetrForSegmentation` in this PR (and perhaps rename the PR). And how can I create an alias for `DetrForSegmentation` (to be renamed to `DetrForImageSegmentation`)?<|||||>Let's merge this PR as-is and update the integration test in another PR.

For the alias you can simply do `DetrForImageSegmentation = DetrForSegmentation`