code: def __len__(self):
      return len(self.dataset)
  
imports: import time
  import unittest
  import json
  import os
  import random
  from pgl.utils.data.dataset import Dataset, StreamDataset
  from pgl.utils.data.dataloader import Dataloader
label_desc: Use image node_id 3 for calling the ListDataset obj's underlying member method code with example usage: obj.__len__() and returns: len