code: def __len__(self):
      return self.len
  
imports: import torch
  import os
  from torch.utils.data import DataLoader, Dataset
  from enum import IntEnum
  from random import choice
  import random
  import collections
  import time
  from turing.utils import namedtorchbatch
  from turing.text import mask, torch_long, PAD
  from turing.sources import QueryPassageDataset, QueryInstanceDataset, PretrainingDataCreator, TokenInstance, QueryPassageFineTuningDataset, WikiNBookCorpusPretrainingDataCreator, CleanBodyDataCreator, NumpyPretrainingDataCreator
  from turing.sources import WikiPretrainingDataCreator
  from pytorch_pretrained_bert.tokenization import BertTokenizer
label_desc: Use image node_id 2 for calling the QADataset obj's underlying member method code with example usage: obj.__len__() and returns: self