code: def export_graph(checkpoint_path, output_nodes):
      
      if not tf.gfile.Exists(checkpoint_path):
          raise AssertionError(
              "Export directory doesn't exists. Please specify an export "
              "directory: %s" % checkpoint_path)
  
      if not output_nodes:
          print("You need to supply the name of a node to --output_node_names.")
          return -1
  
          # We retrieve our checkpoint fullpath
      checkpoint = tf.train.get_checkpoint_state(checkpoint_path)
      input_checkpoint = checkpoint.model_checkpoint_path
  
      # We precise the file fullname of our freezed graph
      output_graph = checkpoint_path + "/frozen_model.pb"
  
      # We clear devices to allow TensorFlow to control on which device it will load operations
      clear_devices = True
  
      # We start a session using a temporary fresh Graph
      with tf.Session(graph=tf.Graph()) as sess:
          # We import the meta graph in the current default Graph
          saver = tf.train.import_meta_graph(input_checkpoint + '.meta', clear_devices=clear_devices)
  
          # We restore the weights
          saver.restore(sess, input_checkpoint)
  
          # We use a built-in TF helper to export variables to constants
          output_graph_def = tf.graph_util.convert_variables_to_constants(
              sess,  # The session is used to retrieve the weights
              tf.get_default_graph().as_graph_def(),  # The graph_def is used to retrieve the nodes
              output_nodes  # The output node names are used to select the useful nodes
          )
  
          # Finally we serialize and dump the output graph to the filesystem
          with tf.gfile.GFile(output_graph, "wb") as f:
              f.write(output_graph_def.SerializeToString())
          print("%d ops in the final graph." % len(output_graph_def.node))
  
      return output_graph_def
docs: Export a graph stored in a checkpoint as a *.pb file.
      :param checkpoint_path: The checkpoint path which should be frozen.
      :param output_nodes: The output nodes you care about as a list of strings (their names).
      :return: