[Docs] Add docstring and type hint for initialization function of DDPM Scheduler

[curiosity654]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Docstring and type hint miss in initialization function of DDPM scheduler

Modification

as title

[CLAassistant]

All committers have signed the CLA.

[LeoXing1996]

@curiosity654 Thanks for your contribution! This PR looks good to me.

[OpenMMLab-Assistant001]

Hi @curiosity654 ！First of all, we want to express our gratitude for your significant PR in the MMEditing. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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