[docs] unet type hints

[a-r-r-o-w]

What does this PR do?

Addresses #7044 (comment) along with some other minor type annotation changes.

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@yiyixuxu

[a-r-r-o-w]

this has been done to match the docs @yiyixuxu. if we don't want it to be auto by default, i can update the docs accordingly

[yiyixuxu]

I think it's ok

[yiyixuxu]

thanks!

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[stevhliu]

Nice, thanks for adding!