FlowMatch schedulers - closing the gap

[vladmandic]

As stated here, lets close the scheduler gap!

Problem statement:

• Most (if not all quality ones) new models are DiT based
• Implementation of DiT based models in Diffusers typically requires Flow-match type schedulers
• Currently only working scheduler is FlowMatchEulerDiscreteScheduler which creates a massive gap!

(just for completeness, there is also an older implementation of FlowMatchHeunDiscreteScheduler, but its not updated to support required set_timesteps or mu inputs, so it cannot be used with newer DiT models like Flux.1)

on the other hand, ComfyUI allows use of any regular scheduler with DiT and does not require separate duplicate -FlowMatch variant of the same scheduler (e.g. EulerDiscrete vs FlowMatchEulerDiscrete) with Flux.1.

reference: matrix of different schedulers used with Flux.1.Dev via ComfyUI:
https://civitai.com/articles/6582/flux1-dev-sampler-scheduler-comparison

cc: @yiyixuxu @sayakpaul @DN6 @asomoza

[yiyixuxu]

cc @hlky here too if interested!

[hlky]

I'd have to dig into FlowMatch a bit more in the community implementations, but it looks like there's little difference between EulerDiscrete and FlowMatchEulerDiscrete so FlowMatchEulerDiscrete could be integrated into EulerDiscrete controlled by some configuration option, this would avoid the need for separate duplicate variants and reduce maintenance burden - as there's already some divergence between schedulers, in set_timesteps for example, where some support sigmas/timesteps as input and others don't, support for final_sigmas_type, etc. Schedulers as a whole could probably do with an overhaul.