message: add the missing _transformation_name method
diff: @@ -5280,6 +5280,9 @@ class _GroupByWindowDataset(UnaryDataset):
  def _functions(self):
  return [self._key_func, self._reduce_func, self._window_size_func]
  + def _transformation_name(self):
  + return "Dataset.group_by_window()"
  +
  def _collect_resource_inputs(op):
  """Collects resource inputs for the given ops (and its variant inputs)."""
  