code: def encode(self, destination, field, malloc):
      padded_field = (field + self.pad_char * self.max_len)[
          : self.max_len
      ]
      field = np.frombuffer(padded_field.encode("ascii"), dtype="uint8")
      return super().encode(destination, field, malloc)
  
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 StringField obj's underlying member method code with example usage: obj.encode(destination, field, malloc) and returns: super