class_docstr: Allow per-line lazy access to multiple text files using numpy memmap.
code: def __len__(self):
      return self._size
  
imports: import datetime
  import json
  import multiprocessing
  import os
  import pickle
  import time
  from functools import partial
  from typing import Callable, List, Optional, Type
  import numpy
  import torch
  from nemo.core import Dataset
  from nemo.utils import AppState, logging
label_desc: Use image node_id 3 for calling the TextMemMapDataset obj's underlying member method code with example usage: obj.__len__() and returns: self