[fix] Fix latent handoff and FastWan-QAD example path

[aryan5v]

Summary

• return latent worker outputs on CPU before sending them to the parent process, avoiding CUDA IPC for external decode paths
• point the FastWan-QAD TAEHV example at the published FastVideo/FastWan-QAD-1.3B pipeline and existing transformer checkpoint path

Testing

• python3 -m py_compile fastvideo/tests/worker/test_multiproc_executor.py fastvideo/worker/multiproc_executor.py examples/inference/optimizations/FastWan_QAD_TAEHV.py

Closes #1524

[aryan5v]

Rebased onto latest main, dropped the superseded fastvideo/platforms/cuda.py SM120 fallback, and pushed the cleaned branch under fix-latent-handoff-fastwan-qad-path.

The PR now keeps only the two still-needed fixes:

• CPU handoff for latent worker outputs before returning to the parent process
• FastWan-QAD TAEHV example defaults/path fix

Validation: python3 -m py_compile fastvideo/tests/worker/test_multiproc_executor.py fastvideo/worker/multiproc_executor.py examples/inference/optimizations/FastWan_QAD_TAEHV.py.

[mergify]

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[gemini-code-assist]

Code Review

This pull request updates the FastWan_QAD_TAEHV.py example script with correct usage instructions and updated model defaults. It also introduces a helper function _prepare_worker_output_for_parent in multiproc_executor.py to move latent CUDA tensors to the CPU before sending them to the parent process, avoiding CUDA IPC issues for small latent outputs, and adds corresponding unit tests. The reviewer suggested improving the robustness of this helper function by handling cases where fastvideo_args might be None or lack the output_type attribute.

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[mergify]

Pre-commit checks failed

Hi @aryan5v, the pre-commit checks have failed. To fix them locally:

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pre-commit run --all-files

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[mergify]

This PR has merge conflicts with the base branch. Please rebase:

git fetch origin main
git rebase origin/main
# Resolve any conflicts, then:
git push --force-with-lease