Support Wan nemo-ci long convergence tests

[huvunvidia]

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[claude]

Light code review - LGTM with minor notes. The change is correct - both WanDatasetConfig.path and FluxDatasetConfig.path exist as dataclass fields. Observations: (1) PR description template not filled in. (2) Consider adding a unit test for the new diffusion_dataset_path CLI arg. (3) CI skips functional tests for perf-scripts-only changes. Suggested test cases: No perf tests impacted.