class_docstr: Only calculate the unknown region if trimap provided.
  Refer to: https://github.com/open-mlab/mmediting/blob/master/mmedit/core/evaluation/metrics.py
code: def gauss_gradient(self, img, sigma):
      filter_x, filter_y = self.gauss_filter(sigma)
      img_filtered_x = cv2.filter2D(
          img, -1, filter_x, borderType=cv2.BORDER_REPLICATE
      )
      img_filtered_y = cv2.filter2D(
          img, -1, filter_y, borderType=cv2.BORDER_REPLICATE
      )
      return np.sqrt(img_filtered_x**2 + img_filtered_y**2)
  
imports: import cv2
  import numpy
  from scipy.ndimage.filters import convolve
  from scipy.special import gamma
  from skimage.measure import label
label_desc: Use image node_id 5 for calling the Grad obj's underlying member method code with example usage: obj.gauss_gradient(img, sigma) and returns: np