Apply series name to result of SeriesGroupby.apply() (rapidsai#8939)

Closes rapidsai#8899 

Applies the name of `SeriesGroupby.obj` to the results of a `SeriesGroupby.apply()` operation; originally, this would be left as `None`.

Authors:
  - Charles Blackmon-Luca (https://github.com/charlesbluca)

Approvers:
  - Sheilah Kirui (https://github.com/skirui-source)
  - Michael Wang (https://github.com/isVoid)

URL: rapidsai#8939

python/cudf/cudf/core/groupby/groupby.py

@@ -1222,6 +1222,14 @@ def agg(self, func):
 
         return result
 
+    def apply(self, func):
+        result = super().apply(func)
+
+        # apply Series name to result
+        result.name = self.obj.name
+
+        return result
+
 
 class Grouper(object):
     def __init__(self, key=None, level=None):

python/cudf/cudf/tests/test_groupby.py

@@ -2104,3 +2104,13 @@ def test_groupby_first(data, agg):
     expect = pdf.groupby("a").agg(agg)
     got = gdf.groupby("a").agg(agg)
     assert_groupby_results_equal(expect, got, check_dtype=False)
+
+
+def test_groupby_apply_series_name():
+    def foo(x):
+        return x.sum()
+
+    got = make_frame(DataFrame, 3).groupby("x").y.apply(foo)
+    expect = make_frame(pd.DataFrame, 3).groupby("x").y.apply(foo)
+
+    assert expect.name == got.name