title: freeze_graph in tensorflow: AssertionError: y_ is not in graph
body: <p>In Tensorflow, after training the model, I saved it use:</p>
  
  <pre><code>with tf.Session() as session:
  /** 
      ------- Model training code goes here ------
  **/
  tf.train.write_graph(session.graph_def, '.', '../har.pbtxt')  
  saver.save(session,save_path = "../har.ckpt")
  </code></pre>
  
  <p>And to freeze and save the optimized model:</p>
  
  <pre><code>from tensorflow.python.tools import freeze_graph
  from tensorflow.python.tools import optimize_for_inference_lib
  
  freeze_graph.freeze_graph(input_graph = "../har.pbtxt",  input_saver = "",
               input_binary = False, input_checkpoint = "../har.ckpt", output_node_names = "y_",
               restore_op_name = "save/restore_all", filename_tensor_name = "save/Const:0",
               output_graph = "frozen_har.pb", clear_devices = True, initializer_nodes = "")
  
  input_graph_def = tf.GraphDef()
  with tf.gfile.Open(output_frozen_graph_name, "r") as f:
      data = f.read()
      input_graph_def.ParseFromString(data)
  
  output_graph_def = optimize_for_inference_lib.optimize_for_inference(
          input_graph_def,
          ["input"], 
          ["y_"],
          tf.float32.as_datatype_enum)
  
  f = tf.gfile.FastGFile("optimized_frozen_har.pb", "w")
  f.write(output_graph_def.SerializeToString())
  </code></pre>
  
  <p>However, I get the error:</p>
  
  <blockquote>
    <p>Traceback (most recent call last):<br>
    File
    "C:\Users\asus\Desktop\cnn.py", line 176, in 
        output_graph = "frozen_har.pb", clear_devices = True, initializer_nodes = "")<br>
    File
    "C:\Users\asus\AppData\Local\Programs\Python\Python35\lib\site-packages\tensorflow\python\tools\freeze_graph.py",
    line 122, in freeze_graph
        variable_names_blacklist=variable_names_blacklist)   File "C:\Users\asus\AppData\Local\Programs\Python\Python35\lib\site-packages\tensorflow\python\framework\graph_util_impl.py",
    line 202, in convert_variables_to_constants
        inference_graph = extract_sub_graph(input_graph_def, output_node_names)<br>
    File
    "C:\Users\asus\AppData\Local\Programs\Python\Python35\lib\site-packages\tensorflow\python\framework\graph_util_impl.py",
    line 141, in extract_sub_graph
        assert d in name_to_node_map, "%s is not in graph" % d AssertionError: y_ is not in graph</p>
  </blockquote>
  
  <p>I defined <code>y_</code>in my code as the output:</p>
  
  <pre><code>y_ = tf.nn.softmax(tf.matmul(f, out_weights) + out_biases)
  </code></pre>
  
  <p>What seens to be the problem?</p>