Fix missing load_video documentation and load_video import in WanVideoToVideoPipeline example code

[Meatfucker]

What does this PR do?

As it says in the title, just minor janitorial work. The utils docs does not mention load_video at all, and the example code is missing the import.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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@sayakpaul @stevhliu
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