ENH: Move corrwith from transformation to reduction kernels in groupby.base
- [x] closes #31270
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
this looks correct, but @TomAugspurger if you'd have a look. What's the user-facing change here? Can you add that to the release note? > What's the user-facing change here? Can you add that to the release note?

There change doesn't affect users. However this allows us to make subsequent changes that have benefit users. ping Thanks @fujiaxiang 