Improve docstrings and type hints in scheduling_ddim_parallel.py

[delmalih]

What does this PR do?

This PR improves the docstrings and type hints in src/diffusers/schedulers/scheduling_ddim_parallel.py to follow the project conventions as described in #9567.

Context

This is part of a broader effort to improve documentation across all scheduler files in src/diffusers/schedulers/. See issue #9567 for details.

Testing

• Code passes make style
• Code passes make quality
• Code passes make fix-copies
• Existing tests still pass

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? issue.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? (N/A - documentation only changes)

Who can review?

cc @stevhliu for review

[stevhliu]

thanks!

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[stevhliu]

i can merge once you run make style !

[delmalih]

@stevhliu done !