code: def join_coupling(z_id, z_tr, split_dim, identity_first):
      """Reverse split function used in coupling flows."""
      assert z_id.shape.rank == 3 and z_tr.shape.rank == 3
      result = [z_id, z_tr] if identity_first else [z_tr, z_id]
      if split_dim == "c":
          result = tf.concat(
              result, axis=2
          )  # concat in the channel dimension
      elif split_dim == "a":
          result = tf.stack(
              result, axis=3
          )  # stack in the channel dimension
      elif split_dim == "t":
          result = tf.stack(
              result, axis=2
          )  # stack in the time dimension
      return result
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import copy
  import math
  from tensor2tensor.layers import common_attention
  from tensor2tensor.layers import common_layers
  from tensor2tensor.models.transformer import transformer_decoder_layer
  import tensorflow.compat.v1
  import tensorflow_probability
label_desc: Use image node_id 15 for calling a global function with example usage: join_coupling(z_id, z_tr, split_dim, identity_first) and returns: result