GitLab

* Enhance docstrings and type hints in PNDMScheduler class

- Updated parameter descriptions to include default values and specific types using Literal for better clarity.
- Improved docstring formatting and consistency across methods, including detailed explanations for the `_get_prev_sample` method.
- Added type hints for method return types to enhance code readability and maintainability.

* Refactor docstring in PNDMScheduler class to enhance clarity

- Simplified the explanation of the method for computing the previous sample from the current sample.
- Updated the reference to the PNDM paper for better accessibility.
- Removed redundant notation explanations to streamline the documentation.