Use distinct_count instead of super().nunique in single_column_frame

Co-authored-by: Ashwin Srinath <[email protected]>

python/cudf/cudf/core/single_column_frame.py

@@ -353,4 +353,4 @@ def nunique(self, method: builtins.str = "sort", dropna: bool = True):
         int
             Number of unique values in the column.
         """
-        return sum(super().nunique(method=method, dropna=dropna).values())
+        return self._column.distinct_count(method=method, dropna=dropna)