code: def update(self, labels, preds):
          
          for label, pred in zip(labels, preds):
              batch_size = label.shape[0]
              # label: (batch_size, 1)
              label = label.as_in_context(pred.context).reshape((label.size,1))
              # mse: (batch_size,)
              mse = mx.nd.square(label - pred)
              # mse: (1,)
              mse = mx.nd.sum(mse)
              self.num_inst += batch_size
  
              self.sum_metric += mse.asscalar()
docs: :param labels: List of (batch_size,)-shaped NDArrays.
          :param preds: List of (batch_size,1)-shaped NDArrays.