Cosmos3 ModularPipeline

[yzhautouskay]

What does this PR do?

Summary

• Add Cosmos3 modular pipeline support via Cosmos3OmniModularPipeline and Cosmos3OmniBlocks.
• Implement modular Cosmos3 stages for encoding, pre-denoise setup, denoising loop, and decoding.
• Register/export Cosmos3 modular pipeline components in modular and top-level package mappings.
• Add Cosmos3 modular documentation and usage section in docs/source/en/api/pipelines/cosmos3.md.
• Add strict elementwise parity tests across text/image/video, optional sound, and action-conditioned modes.

Test Plan

• PYTHONPATH=src python -m pytest -q tests/pipelines/cosmos/test_cosmos3_modular_parity.py -vv

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