code: def __len__(self):
      return len(self.idx2psgs)
  
imports: import multiprocessing
  import os
  import pickle
  import random
  from typing import Optional
  import numpy
  from torch.utils.data import Dataset
  from nemo.collections.common.tokenizers.tokenizer_spec import TokenizerSpec
label_desc: Use image node_id 3 for calling the BertInformationRetrievalDataset obj's underlying member method code with example usage: obj.__len__() and returns: len