Fix: UNet2DModel::__init__ type hints; fixes issue #4806

[fpgaminer]

What does this PR do?

Fixes type hints on UNet2DModel::init. Tuples were specified as e.g. Tuple[str] when they should be Tuple[str, ...] to specify that they can be tuples of any size rather than a tuple of size exactly 1.

Fixes # 4806

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Who can review?

[yiyixuxu]

thank you!

[HuggingFaceDocBuilderDev]

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[stevhliu]

LGTM, thanks!