PERF: groupby.transform skip fast path when only a single group exists
- [x] closes #44977
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.


groupby.transform can skip the fast path evaluation when only a single group exists. See additional comments in #44977. 

Note: one test had to be updated as it applied a groupby on a single group and expected a warning that is only generated by the fast path. The test was updated to include multiple groups so that the fast path would get evaluated. 

thanks @lukemanley 