code: def mse(predicted, actual):
      
      diff = predicted - actual
      return np.average(diff * diff, axis=0)
docs: Mean squared error of predictions.
  
      .. versionadded:: 0.5.0
  
      Parameters
      ----------
      predicted : ndarray
          Predictions on which to measure error. May
          contain a single or multiple column but must
          match `actual` in shape.
  
      actual : ndarray
          Actual values against which to measure predictions.
  
      Returns
      -------
      err : ndarray
          Mean squared error of predictions relative to actual
          values.