code: def __len__(self):
      return len(self._index)
  
imports: import os
  import shutil
  import struct
  from functools import lru_cache
  from itertools import accumulate
  import numpy
  import torch
label_desc: Use image node_id 6 for calling the MMapIndexedDataset obj's underlying member method code with example usage: obj.__len__() and returns: len