Fix return type of DataFrame.argsort (#7706)

Fixes: #7577 

This PR fixes the return type of `DataFrame.argsort` to return `cudf.Series` instead of a `NumericalColumn`.

Authors:
  - GALI PREM SAGAR (@galipremsagar)

Approvers:
  - Keith Kraus (@kkraus14)

URL: #7706

python/cudf/cudf/core/dataframe.py

@@ -3841,10 +3841,32 @@ def argsort(self, ascending=True, na_position="last"):
         - Support axis='index' only.
         - Not supporting: inplace, kind
         - Ascending can be a list of bools to control per column
+
+        Examples
+        --------
+        >>> import cudf
+        >>> df = cudf.DataFrame({'a':[10, 0, 2], 'b':[-10, 10, 1]})
+        >>> df
+            a   b
+        0  10 -10
+        1   0  10
+        2   2   1
+        >>> inds = df.argsort()
+        >>> inds
+        0    1
+        1    2
+        2    0
+        dtype: int32
+        >>> df.take(inds)
+            a   b
+        1   0  10
+        2   2   1
+        0  10 -10
         """
-        return self._get_sorted_inds(
+        inds_col = self._get_sorted_inds(
             ascending=ascending, na_position=na_position
         )
+        return cudf.Series(inds_col)
 
     @annotate("SORT_INDEX", color="red", domain="cudf_python")
     def sort_index(

python/cudf/cudf/tests/test_dataframe.py

@@ -8495,3 +8495,24 @@ def test_explode(data, labels, ignore_index, p_index, label_to_explode):
     got = gdf.explode(label_to_explode, ignore_index)
 
     assert_eq(expect, got, check_dtype=False)
+
+
+@pytest.mark.parametrize(
+    "df,ascending,expected",
+    [
+        (
+            cudf.DataFrame({"a": [10, 0, 2], "b": [-10, 10, 1]}),
+            True,
+            cudf.Series([1, 2, 0], dtype="int32"),
+        ),
+        (
+            cudf.DataFrame({"a": [10, 0, 2], "b": [-10, 10, 1]}),
+            False,
+            cudf.Series([0, 2, 1], dtype="int32"),
+        ),
+    ],
+)
+def test_dataframe_argsort(df, ascending, expected):
+    actual = df.argsort(ascending=ascending)
+
+    assert_eq(actual, expected)