Cosmos3 Distilled support

[yzhautouskay]

What does this PR do?

Cosmos3 distilled (few-step) support

Adds inference support for the distilled few-step Cosmos3 checkpoints through the
modular pipeline (Cosmos3OmniModularPipeline).

Checkpoints supported

• nvidia/Cosmos3-Super-Text2Image-4Step (distilled T2I)
• nvidia/Cosmos3-Super-Image2Video-4Step (distilled I2V)

Both ship a FlowMatchEulerDiscreteScheduler with a fixed sigma schedule
(scheduler.config.fixed_step_sampler_config.t_list) and bake classifier-free
guidance into the weights. They are only supported via the modular pipeline; the
task-based Cosmos3OmniPipeline does not implement the distilled contract.

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[yiyixuxu]

thanks!
i left some comments

[github-actions]

Hi @yzhautouskay, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.

[yzhautouskay]

thanks @yiyixuxu! I've refactored it to have separate Cosmos3DistilledModularPipeline

[yiyixuxu]

@yzhautouskay can you resolve the conflict?

[yiyixuxu]

thanks, I left some comments

[yiyixuxu]

can you update the test to use ModularPipelineTesterMixin?
https://github.com/huggingface/diffusers/blob/main/tests/modular_pipelines/flux2/test_modular_pipeline_flux2_klein.py

[yzhautouskay]

Done, distilled tests now use ModularPipelineTesterMixin. For now tiny fixture lives under my personal account at yzhautouskay/tiny-cosmos3-distilled-modular-pipe
Could you create hf-internal-testing/tiny-cosmos3-distilled-modular-pipe and copy it over ? I'll then flip the TINY_DISTILLED_REPO constant in the test (and pretrained_model_name_or_path will need to be changed for components in modular_model_index.json)