Fix attention backend leaking across models

[raghulchandramouli]

What does this PR do?

In a nutshell, It Prevents ModelMixin.set_attention_backend() from changing the process-global active attention backend. The method now only configures attention processors belonging to the target model.

Self-review:

Ran self-review and these are the validation I got from it:

• Blocking issues: None
• Non-blocking issues: None
• Dead code: None
• Documentation changes required: None

Test I Ran:

• Regression tests for Flux and QwenImage: all passed
• make style: passed
• make fix-copies: passed
• git diff --check: passed

Test I wrote:

• Added a regression test in tests/attention.py to make sure that ModelMixin.set_attention_backend() does not modify the registry.

discussion

This appears as an issue raised by sayak Paul - #14249

Fixes #14249

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It would be great if @qgallouedec or @sayakpaul can you take a quick review of this PR??

[DN6]

Thanks 👍🏽

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[raghulchandramouli]

hello @DN6 both of these issues being raised are not becuz of me, its an Anima mismatch error which was never touched by me

[raghulchandramouli]

Hi @DN6, I updated this PR to the latest main, but two unrelated CI jobs
remain failing:

• Fast PyTorch Modular Pipeline CPU tests: only
  TestQwenImageEditModularPipelineFast::test_num_images_per_prompt timed out
  after 60 seconds; 648 tests passed.
• Torch CUDA Tests (models): 22 MiniMax-H3 tests failed. These failures
  are being addressed separately in [tests] fix minimax h3 test failures #14421.

This PR does not modify MiniMax-H3 or modular pipeline code. GitHub does not
allow me to rerun the jobs because rerunning requires repository admin
access.

Could you please rerun the failed jobs, or advise whether the MiniMax-H3
failure can be ignored until #14421 is merged? Thanks!