class_docstr: Only calculate the unknown region if trimap provided.
  Refer to: Refer to: https://github.com/open-mlab/mmediting/blob/master/mmedit/core/evaluation/metrics.py
code: def __init__(self):
      self.conn_diffs = 0
      self.count = 0
  
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 1 to create a new Conn object  with example: obj = Conn()