Fix `groupby` + `reduce` for partitions with only 1 row (#795)
Fix `groupby` + `reduce` for partitions with only 1 row (#795)

* Fix `groupby` + `reduce` for partitions with only 1 row

* Resolves #747
* No longer assume more than one row in the data
* Change the squeeze on the `by` parameter to only squeeze along the
  opposite axis as the groupby.

* Lint

modin-project/modin,modin-project/modin