Fix PIL backend fallback when torchvision is unavailable

[ErenAta16]

This PR fixes a regression where PIL-based image/video processors were incorrectly treated as requiring torchvision.

As a result, AutoProcessor / AutoImageProcessor could fail in environments without torchvision, even though a valid PIL fallback exists.

What changed

• Updated import-structure backend resolution in import_utils so PIL modules do not inherit torchvision requirements:
  • image_processing_pil_*
  • video_processing_pil_*
• Added a normalization step to prevent torchvision from being attached to PIL-only modules during backend aggregation.
• Added regression tests for:
  • PIL import-structure backend correctness
  • Auto-backend fallback to PIL when torchvision is unavailable

Fixes #45042

Tests

• python -m pytest tests/utils/test_import_structure.py -k "pil_import_structure_does_not_require_torchvision" -q
• python -m pytest tests/models/auto/test_image_processing_auto.py -k "auto_backend_falls_back_to_pil_when_torchvision_is_unavailable or backend_kwarg_pil" -q

[ErenAta16]

Thanks for the run. The current failure is in
tests/models/prophetnet/test_modeling_prophetnet.py::ProphetNetModelTest::test_encoder_decoder_model_generate,
which is outside this PR scope.

This PR only changes PIL backend import-structure/fallback logic:

• src/transformers/utils/import_utils.py
• tests/utils/test_import_structure.py
• tests/models/auto/test_image_processing_auto.py

Could we rerun tests_torch once? If the same ProphetNet test is consistently failing, I can help track it separately.

[ArthurZucker]

Ty! there's #45045 which came first and is more aligned with what we are trying to do!
Tho checking for torch "torch" in the file might also just work. We could do the same for PIL that need torch

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

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View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=45060&sha=8260c8