code: def sequence_key(sequence):
      """Returns a key for mapping sequence paths to graph vertices."""
      return ":".join([str(s) for s in sequence])
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from collections import deque
  import glob
  import os
  import shutil
  import time
  import numpy
  from six.moves import range
  from tensor2tensor.bin import t2t_trainer
  from tensor2tensor.data_generators import text_encoder
  from tensor2tensor.insights import graph
  from tensor2tensor.insights import query_processor
  from tensor2tensor.utils import decoding
  from tensor2tensor.utils import trainer_lib
  from tensor2tensor.utils import usr_dir
  import tensorflow.compat.v1
  from tensorflow.python import debug
label_desc: Use image node_id 4 for calling a global function with example usage: sequence_key(sequence) and returns: str