title: Graph is finalized and cannot be modified
body: <p><a href="https://i.stack.imgur.com/ew6kR.png" rel="nofollow noreferrer">My Linux terminal screenshot here</a></p>
  
  <pre><code># the error entry in linux terminal screenshot            
  if FLAGS.validation_db:
        val_model.start_queue_runners(sess)
        Validation(sess, val_model, 0)
  
  #the call func above val_model
  def start_queue_runners(self, sess):
      logging.info('Starting queue runners (%s)', self.stage)
      # Distinguish the queue runner collection (for easily obtaining them by 
      collection key)
      queue_runners = tf.get_collection(tf.GraphKeys.QUEUE_RUNNERS, 
      scope=self.stage+'.*')
      for qr in queue_runners:
          if self.stage in qr.name:
              tf.add_to_collection(digits.GraphKeys.QUEUE_RUNNERS, qr)
  
      self.queue_coord = tf.train.Coordinator()
      self.queue_threads = tf.train.start_queue_runners(sess=sess, 
      coord=self.queue_coord,
  
      collection=digits.GraphKeys.QUEUE_RUNNERS)
      logging.info('Queue runners started (%s)', self.stage)
  </code></pre>
  
  <p>what happend in my linux-terminal，where is need to modify?
  i find some answer about it in stackoverflow,but its not sut me</p>
  
  <p>the specific error under:</p>
  
  <pre><code>2017-12-19 05:49:34 [INFO] Starting queue runners (val)
  Traceback (most recent call last):
  File "/root/digits/digits/tools/tensorflow/main.py", line 627, in main
  val_model.start_queue_runners(sess)
  File "/root/digits/digits/tools/tensorflow/model.py", line 208, in 
  start_queue_runners
  tf.add_to_collection(digits.GraphKeys.QUEUE_RUNNERS, qr)
  File "/usr/local/lib/python2.7/dist-
  packages/tensorflow/python/framework/ops.py", line 4
  248, in add_to_collection    get_default_graph().add_to_collection(name, 
  value)
  File "/usr/local/lib/python2.7/dist-
  packages/tensorflow/python/framework/ops.py", line 2
  792, in add_to_collection    self._check_not_finalized()
  File "/usr/local/lib/python2.7/dist-
  packages/tensorflow/python/framework/ops.py", line 2
  181, in _check_not_finalized    raise RuntimeError("Graph is finalized and 
  cannot be modified.")
  RuntimeError: Graph is finalized and cannot be modified.
  </code></pre>