[docs] add docstrings in pipline_stable_diffusion.py

[jeongiin]

What does this PR do?

Fixes : #9567

I have added a docstring to the rescale_noise_cfg function. I plan to continue addressing this issue, and I would greatly appreciate any advice you could offer on writing effective commit messages or PRs. Thank you!

ps. I've worked on other tasks as well, so there are many commit messages, but only one file was actually changed.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@a-r-r-o-w @stevhliu