code: def custom_decoder(sample):
      for key, value in sample.items():
          if key == "png":
              # check that images have already been decoded
              assert not isinstance(value, bytes)
          elif key.endswith("custom"):
              sample[key] = "custom-value"
      return sample
  
file_path: ray/python/ray/data/tests/test_webdataset.py
imports: import glob
  import io
  import os
  import tarfile
  import pytest
  import ray
  from ray.tests.conftest import *
label_desc: Use image node_id 4 for calling a global function with example usage: custom_decoder(sample) and returns: sample