DDPMScheduler claims to have set_alpha_to_one in combination with steps_offset

When inspecting the configuration for SD/SDXL we noticed that `set_alpha_to_one` is mentioned in the scheduler's init docstring but it is nowhere to be found. It seems that DDPM either lacks this functionality, or does not need it.

Start with the DDPMScheduler init docstring and its configuration handling, then compare the SD/SDXL scheduler configurations that mention set_alpha_to_one alongside steps_offset. Determine whether the setting is missing functionality or stale documentation, and confirm the chosen resolution with focused scheduler tests or documentation checks.

索引モデルが issue の本文から書いたものです。