Raise NotImplementedError for MultiIndex.to_series (#14049)

Fixes #14035 

This PR raises an error for `MultiIndex.to_series` because we cannot store `tuple` type columns in `cudf`.

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Lawrence Mitchell (https://github.com/wence-)

URL: #14049

python/cudf/cudf/core/multiindex.py

@@ -219,6 +219,12 @@ def names(self, value):
             )
         self._names = pd.core.indexes.frozen.FrozenList(value)
 
+    @_cudf_nvtx_annotate
+    def to_series(self, index=None, name=None):
+        raise NotImplementedError(
+            "MultiIndex.to_series isn't implemented yet."
+        )
+
     @_cudf_nvtx_annotate
     def astype(self, dtype, copy: bool = True):
         if not is_object_dtype(dtype):

python/cudf/cudf/tests/test_multiindex.py

@@ -1920,3 +1920,9 @@ def test_multiindex_sort_index_partial(levels):
     expect = df.sort_index(level=levels, sort_remaining=True)
     got = cdf.sort_index(level=levels, sort_remaining=True)
     assert_eq(expect, got)
+
+
+def test_multiindex_to_series_error():
+    midx = cudf.MultiIndex.from_tuples([("a", "b")])
+    with pytest.raises(NotImplementedError):
+        midx.to_series()