code: def __len__(self):
      return len(self.pgl_graph_list)
  
imports: import sys
  import os
  import random
  import pgl
  from pgl.utils.logger import log
  from pgl.graph import Graph, MultiGraph
  import numpy
  import pickle
label_desc: Use image node_id 4 for calling the Dataset obj's underlying member method code with example usage: obj.__len__() and returns: len