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* Improve docstrings and type hints in scheduling_ddim.py

- Add complete type hints for all function parameters
- Enhance docstrings to follow project conventions
- Add missing parameter descriptions

Fixes #9567

* Enhance docstrings and type hints in scheduling_ddim.py

- Update parameter types and descriptions for clarity
- Improve explanations in method docstrings to align with project standards
- Add optional annotations for parameters where applicable

* Refine type hints and docstrings in scheduling_ddim.py

- Update parameter types to use Literal for specific string options
- Enhance docstring descriptions for clarity and consistency
- Ensure all parameters have appropriate type annotations and defaults

* Apply review feedback on scheduling_ddim.py

- Replace "prevent singularities" with "avoid numerical instability" for better clarity
- Add backticks around `alpha_bar` variable name for consistent formatting
- Convert Imagen Video paper URLs to Hugging Face papers references

* Propagate changes using 'make fix-copies'

* Add missing Literal