Improve LCMScheduler (#5681)
* Refactor LCMScheduler.step such that prev_sample == denoised at the last timestep in the schedule.
* Make timestep scaling when calculating boundary conditions configurable.
* Reparameterize timestep_scaling to be a multiplicative rather than division scaling.
* make style
* fix dtype conversion
* make style
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Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
