Fix unguarded torchvision import in Cosmos

[DN6]

What does this PR do?

Noticed this is resulting in a failure on our PyPi release test because the torchvision import is unguarded
https://github.com/huggingface/diffusers/actions/runs/22723437757/job/65891766459#step:7:408

This PR removes the bare import for torchvision and torchvision.transforms (they're not used anywhere in the pipeline) and guard the torchvision.transforms.functional

Fixes # (issue)

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