Sync Cosmos3 Diffusers converter support

[lfengad]

Summary

• support Cosmos3 checkpoints with optional action and sound generation
• support current and legacy AVAE/tokenizer layouts and framework-native VLM config layouts
• make language-model remapping reject collisions and exclude visual weights from the text transformer
• support both legacy and current action projection module names
• add --skip-vision-encoder and make --config-only work with an empty output directory
• generate a top-level safetensors index that matches the files actually exported
• keep the converter implementation aligned with the corresponding Cosmos3 package implementation, apart from repository-specific imports and licensing
• add a GPU CI regression that converts the staged Nano DCP, validates the complete Diffusers output against the public Nano reference indexes, and always removes the generated output

Validation

• converted a Cosmos3-Nano DCP checkpoint to a complete Diffusers checkpoint
• verified all 10 exported safetensors files byte-for-byte against nvidia/Cosmos3-Nano
• verified the unified index contains 1,165 tensors with no dangling file references
• loaded the output with Cosmos3OmniPipeline and ran one-step video-plus-sound inference with finite outputs
• verified pytest collection selects the new Nano Diffusers conversion test in the generator GPU workflow
• Ruff check, py_compile, YAML parse, and git diff --check pass