class_docstr: Abstract class for bridge tests
class_docstr_tok: ["Abstract","class","for","bridge","tests"]
docstr: Creates the bridge class to be tests. Must be implemented by
  child classes
docstr_tok: ["Creates","the","bridge","class","to","be","tests",".","Must","be","implemented","by","child","classes"]
code: def _create_bridge(self):
      """Creates the bridge class to be tests. Must be implemented by
      child classes"""
      raise NotImplementedError()
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from __future__ import unicode_literals
  from collections import namedtuple
  import numpy
  import tensorflow
  from tensorflow.python.util import nest
  from seq2seq.encoders.encoder import EncoderOutput
  from seq2seq.models.bridges import ZeroBridge, InitialStateBridge
  from seq2seq.models.bridges import PassThroughBridge
label_desc: Use image node_id 2 for calling the BridgeTest obj's underlying member method code with example usage: obj._create_bridge() without return types