[scheduler] support custom `timesteps` and `sigmas` (#7817)
* support custom sigmas and timesteps, dpm euler
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Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>
Co-authored-by: Benjamin Bossan <BenjaminBossan@users.noreply.github.com>
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
