code: def nrmse(test, ref, mask=None):
      
  
      test, ref, mask = _preprocess_input(test, ref, mask)
  
      if mask is not None:
          test = mask * test
          ref = mask * ref
  
      num = np.sqrt(mse(test, ref))
      deno = np.sqrt(np.mean((np.square(test))))
  
      return num / deno
docs: Return NRMSE
  
      Parameters
      ----------
      ref : np.ndarray
          the reference image
      test : np.ndarray
          the tested image
      mask : np.ndarray, optional
          the mask for the ROI
  
      Notes
      -----
      Compute the metric only on magnitude.
  
      Returns
      -------
      nrmse: float, the nrmse