code: def __len__(self):
      return self.l
  
file_path: ffcv/tests/test_memory_allocation.py
imports: import numpy
  from assertpy import assert_that
  from torch.utils.data import Dataset
  import logging
  import os
  from assertpy import assert_that
  from tempfile import NamedTemporaryFile
  from ffcv.writer import DatasetWriter
  from ffcv.reader import Reader
  from ffcv.fields import BytesField, IntField
label_desc: Use image node_id 2 for calling the DummyDataset obj's underlying member method code with example usage: obj.__len__() and returns: self