[Feature] Support v_prediction

[okotaku]

Motivation

V Prediction from Progressive Distillation for Fast Sampling of Diffusion Models.

Results (Optional)

stable_diffusion_xl_lora_v_prediction_pokemon_blip

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

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📚 Documentation preview 📚: https://DiffEngine--94.org.readthedocs.build/en/94/