Improve docstrings and type hints in scheduling_ddpm_parallel.py (#13027)
* docs: improve docstring scheduling_ddpm_parallel.py
* Update scheduling_ddpm_parallel.py
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
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Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>