Sgm uniform scheduler for SDXL-Lightning models

[KohakuBlueleaf]

Description

Based on the example code/workflow from https://huggingface.co/ByteDance/SDXL-Lightning
I implement the sgm_uniform scheduler for SDXL-lightning series models (also support the SDXL Lightning LoRA models)

I create a new file for this custom scheduler since I guess we will have more and more custom scheduler in the future.

I don't know if we should make some extension API on it. But I will keep it clean at first.

SDXL Lightning Examples (4step Euler):

SDXL Lightning LoRA + KXL Delta Examples (4step Euler):

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

[MoonRide303]

@KohakuBlueleaf I've tested it, seems to be working fine - results almost identical as from forge and ComfyUI.

photography of a nice girl
Steps: 8, Sampler: Euler, CFG scale: 1, Seed: 0, Size: 896x1152, Model hash: 43f0501ac4, Model: sdxl_lightning_8step, Clip skip: 2, RNG: CPU, Schedule type: sgm_uniform, SGM noise multiplier: True, Version: v1.8.0-229-ga6b5a513

ComfyUI:

Forge:

A1111: