title: tf.GraphKeys.TRAINABLE_VARIABLES on output_graph.pb resulting in empty list
body: <p>I'm trying to extract all the weights/biases from a saved model <code>output_graph.pb</code>.</p>
  
  <p>I read the model:</p>
  
  <pre><code>def create_graph(modelFullPath):
      """Creates a graph from saved GraphDef file and returns a saver."""
      # Creates graph from saved graph_def.pb.
      with tf.gfile.FastGFile(modelFullPath, 'rb') as f:
          graph_def = tf.GraphDef()
          graph_def.ParseFromString(f.read())
          tf.import_graph_def(graph_def, name='')
  
  GRAPH_DIR = r'C:\tmp\output_graph.pb'
  create_graph(GRAPH_DIR)
  </code></pre>
  
  <p>And attempted this hoping I would be able to extract all weights/biases
  within each layer. </p>
  
  <pre><code>with tf.Session() as sess:
      all_vars = tf.get_collection(tf.GraphKeys.TRAINABLE_VARIABLES)
      print (len(all_vars))
  </code></pre>
  
  <p>However, I'm getting a value of 0 as the len. </p>
  
  <p>Final goal is to extract the weights and biases and save it to a text file/np.arrays. </p>