Fix missing self in post-process methods

[framonmar7]

What does this PR do?

This PR fixes a repeated bug across several image processor classes (ConditionalDetrImageProcessorFast, DeformableDetrImageProcessorFast, GroundingDinoImageProcessorFast, Mask2FormerImageProcessorFast, and RTDetrImageProcessorFast).

Several post_process* methods were defined without a self parameter, which would raise a TypeError when called from an object instance.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Vision models: @amyeroberts, @qubvel

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: conditional_detr, deformable_detr, grounding_dino, mask2former, rt_detr

[qubvel]

Thanks for fixing this! I will merge this PR. However, for reference, it's not actually a bug because modular files are not executable files. This is actually a definition to remove these methods in the converted modeling file, so there is no effect on the actual code.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.