code: def get_tensor_device(self, tensor_name):
      
      tensor = self._name_to_tensor(tensor_name)
      if isinstance(tensor, tf.Tensor):
        return tensor.device
      else:  # mtf.Tensor
        return None
docs: The device of a tensor.
  
      Note that only tf tensors have device assignments.
  
      Args:
        tensor_name: a string, name of a tensor in the graph.
  
      Returns:
        a string or None, representing the device name.