Add LoRA Support for UNetSpatioTemporalConditionModel

[sr5434]

What does this PR do?

This PR adds LoRA support to the UNetSpatioTemporalConditionModel class by adding the PeftMixin from the loaders directory.

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@patrickvonplaten @sayakpaul

[sayakpaul]

Are there any LoRA checkpoints yet?

[sr5434]

No, I’m working on that. I will let you know when I have them, though.

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