code: def _stringify(key):
      """Flattens tuple and list keys into strings."""
  
      if isinstance(key, (tuple, list)):
          return "_".join([str(el) for el in key])
      return key
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import collections
  from absl import logging
  from adanet import tf_compat
  from adanet.ensemble.ensembler import Ensemble
  from adanet.ensemble.ensembler import Ensembler
  import tensorflow.compat.v2
label_desc: Use image node_id 1 for calling a global function with example usage: _stringify(key) and returns: key, str