Fix code examples of DETR and YOLOS
# What does this PR do?

This is a follow-up PR of #19205.

YOLOS and DETR share the same postprocessing, hence I've added post_process_object_detection to YOLOS, leveraging Copied from statements.

It also improves the code example of DETR, and adds a better one for YOLOS.
_The documentation is not available anymore as the PR was closed or merged._