Add UFOGenScheduler to Community Examples

[dg845]

What does this PR do?

This PR adds a scheduler for the UFOGen model to the community examples. A separate UFOGen pipeline is not currently implemented because UFOGen is intended to be initialized from a pretrained diffusion model (Stable Diffusion 1.5 is used in the paper) and so UFOGenScheduler is designed to be compatible with existing pipelines such as StableDiffusionPipeline.

Based on #6133. The code in scheduling_ufogen.py is the same as in that PR, except with the diffusers imports changed from relative imports to absolute imports.

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@patrickvonplaten
@patil-suraj
@JunbongJang

[patrickvonplaten]

Could we also add a short entry to https://github.com/huggingface/diffusers/blob/main/examples/community/README.md

[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.

[dg845]

Added an entry into the community README for scheduling_ufogen.py/UFOGenScheduler with a usage example (although the usage example doesn't quite work in the sense that the ufogen_model_id_or_path is just a dummy value rather than pointing to a physical trained UFOGen checkpoint, since I'm not aware of any publicly available such checkpoints).