code: def __getitem__(self, idx):
      return (self.captions[idx],)
  
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 2 for calling the CaptionDataset obj's underlying member method code with example usage: obj.__getitem__(idx) and returns: self