add DPM scheduler with EDM formulation (#7120)
* add DPM scheduler with EDM formulation
* set sigmas in init
* add _compute_sigmas
* Apply suggestions from code review
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>
* address some review comments
* up,
* add tests
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Co-authored-by: Pedro Cuenca <pedro@huggingface.co>
