message: fix return type error
diff: @@ -2783,7 +2783,7 @@ def _export_output_to_tensors(export_output):
  elif isinstance(export_output, export_output_lib.RegressionOutput):
  return [export_output.value]
  elif isinstance(export_output, export_output_lib.PredictOutput):
  - return export_output.outputs.values()
  + return list(export_output.outputs.values())
  else:
  raise ValueError(
  '`export_output` must be have type `ClassificationOutput`, '
  