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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_operation_device(self, operation_name):
 """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. """

    operation = self._name_to_operation(operation_name)
    if isinstance(operation, tf.Operation):
      return operation.device
    else:  # mtf.Operation
      return None