code: def get_operation_device(self, operation_name):
      
      operation = self._name_to_operation(operation_name)
      if isinstance(operation, tf.Operation):
        return operation.device
      else:  # mtf.Operation
        return None
docs: The device of an operation.
  
      Note that only tf operations have device assignments.
  
      Args:
        operation_name: a string, name of an operation in the graph.
  
      Returns:
        a string or None, representing the device name.