[docs] Scheduler features

[stevhliu]

Continuation of #7817 (see comment here) that refactors scheduler features for inference to their own doc. It includes:

• custom timesteps and sigmas showcasing AYS
• Karras sigmas
• rescale_betas_zero_snr

[HuggingFaceDocBuilderDev]

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[yiyixuxu]

cc @Beinsezii here we are adding a general doc page for the scheduler now; anything else we should add here?

[Beinsezii]

I feel like a section on the timestep spacings would be beneficial, especially since they're part of the same paper referenced. The paper recommends trailing which is what I and a lot of others have settled on. trailing is unique in that its mutually exclusive with steps_offset ≥ 1 as well.

[bghira]

a good demonstration of the current generation of models' two primary forms of residual noise would probably be a good idea though i can't think of how to integrate that. i just see it a lot and i think the community needs language to describe it with, and common solutions to try. probably for a separate doc