code: def __len__(self):
      return len(self.captions)
  
file_path: ffcv/tests/test_custom_field.py
imports: import pytest
  import numpy
  from uuid import uuid4
  from ffcv.fields.ndarray import NDArrayField, NDArrayDecoder
  from ffcv.writer import DatasetWriter
  from ffcv.loader import Loader, OrderOption
  from tempfile import NamedTemporaryFile
label_desc: Use image node_id 3 for calling the CaptionDataset obj's underlying member method code with example usage: obj.__len__() and returns: len