code: def __len__(self):
      return len(self.data)
  
imports: import json, logging, pickle
  from mimetypes import knownfiles
  from unittest.mock import NonCallableMagicMock
  import torch
  from torch.utils.data import Dataset
  from collections import namedtuple
label_desc: Use image node_id 2 for calling the GenDataset obj's underlying member method code with example usage: obj.__len__() and returns: len