code: def _references_files_by_shard(refs_dir):
      process_dirs = _process_folders(refs_dir)
      shards = collections.defaultdict(list)
      for d in process_dirs:
          ref_files = tf.gfile.Glob(
              os.path.join(d, REF_SHARD_FILE_PREFIX) + "*"
          )
          for f in ref_files:
              shards[_shard_id_for_file(f)].append(f)
      return shards
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import collections
  import json
  import math
  import os
  import re
  import string
  import tempfile
  import six
  from tensor2tensor.data_generators import generator_utils
  from tensor2tensor.data_generators import problem
  from tensor2tensor.data_generators import text_encoder
  from tensor2tensor.data_generators import tokenizer
  from tensor2tensor.data_generators.wikisum import utils
  from tensor2tensor.layers import modalities
  from tensor2tensor.utils import metrics
  from tensor2tensor.utils import registry
  import tensorflow.compat.v1
label_desc: Use image node_id 5 for calling a global function with example usage: _references_files_by_shard(refs_dir) and returns: shards