[Wan] Fix mask padding in Wan VACE pipeline.

[bennyguo]

What does this PR do?

Fixes #11779

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[a-r-r-o-w]

Thanks @bennyguo, you're absolutely right! I checked again and see that I made a mistake initially: https://github.com/ali-vilab/VACE/blob/0897c6d055d7d9ea9e191dce763006664d9780f8/vace/models/wan/wan_vace.py#L187

Thanks for the fix!

[bennyguo]

@a-r-r-o-w Insane response speed! Cheers :)

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