ENH: Enable DataFrame.groupby(...).transform('size')

[rhshadrach]

df = pd.DataFrame({'a': [1, 1, 2, 2, 2], 'b': [3, 4, 5, 6, 7]})
gb = df.groupby(df["a"])
print(gb.transform('size'))

raises TypeError: Transform function invalid for data types, but I think a reasonable expectation is to have the result of gb.size() broadcasted:

0    2
1    2
2    3
3    3
4    3
dtype: int64

where gb.size() produces

a
1    2
2    3
dtype: int64

[rhshadrach]

Closing as duplicate of #27469