[DETR, YOLOS] Fix device bug
# What does this PR do?

This PR fixes a device bug for DETR and YOLOS's `post_process_object_detection` methods, which currently give a device mismatch between CPU and CUDA when running the model on CUDA.

The PR also makes sure the postprocess methods are tested in the integration tests.
_The documentation is not available anymore as the PR was closed or merged._