ENH: Move "corrwith" from transformation_kernels to reduction_kernels in groupby.base

[fujiaxiang]

Code Sample

>>> import pandas as pd
>>> pd.__version__
'1.0.0rc0+183.g4da554f75'
>>> df = pd.DataFrame(
...     {
...         "A": [1, 1, 1, 1],
...         "B": [0, 1, 2, 3]
...     }
... )

>>> df
   A  B
0  1  0
1  1  1
2  1  2
3  1  3

>>> df.groupby("A").corrwith(df)  # Clearly corrwith is a reduction rather than transformation
     B   A
A
1  1.0 NaN

On current master (1.0.0rc0+183.g4da554f75) we have "corrwith" in the list of transformation_kernels rather than reduction_kernels within groupby.base.

We probably have to move it and fix any impact it has.

[fujiaxiang]

take