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 gaussian(self, x, sigma):
      return np.exp(-(x**2) / (2 * sigma**2)) / (
          sigma * np.sqrt(2 * np.pi)
      )
  
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 2 for calling the Grad obj's underlying member method code with example usage: obj.gaussian(x, sigma) and returns: unknown