code: def __len__(self):
      return self.l
  
file_path: ffcv/tests/test_webdataset.py
imports: from os import path
  from glob import glob
  import tempfile
  import numpy
  from tempfile import TemporaryDirectory, NamedTemporaryFile
  import torch
  from torch.utils.data import Dataset
  import webdataset
  from ffcv import DatasetWriter
  from ffcv.reader import Reader
  from ffcv.fields import IntField, FloatField
  from test_writer import validate_simple_dataset
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