code: def __init__(self, max_len: int, pad_char="\0"):
      self.max_len = max_len
      self.pad_char = pad_char
      super().__init__(np.dtype("uint8"), (max_len,))
  
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 1 to create a new StringField object from inherited base classes: NDArrayField with example: obj = StringField(max_len, pad_char)