Improve docstrings and type hints in scheduling_ddim.py

[delmalih]

What does this PR do?

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

Changes made:

First commit: Initial improvements

• Added complete type hints for all function parameters and return types
• Enhanced docstrings to follow project conventions
• Added missing parameter descriptions

Second commit: Further enhancements

• Updated parameter types and descriptions for improved clarity
• Refined method docstrings explanations to better align with project standards
• Added optional annotations for parameters where applicable

Summary of improvements:

• ✅ Complete type hint coverage across all functions
• ✅ Consistent docstring formatting with Args and Returns sections
• ✅ Clear and comprehensive parameter descriptions
• ✅ Proper use of optional type annotations
• ✅ Full alignment with project documentation standards

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
• 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 @yiyixuxu @stevhliu @a-r-r-o-w for review