title: Tensorflow: ValueError: Can't load save_path when it is None in single shot detection
body: <p>I have just started working with tensorflow in python. I am trying to train Single shot detection using tensorflow for pascalvoc dataset. While creating tfrecords and during evaluation using VGG_VOC0712_SSD_300x300_ft_iter_120000.ckpt trained model there was no error. Whereas, when I am trying to train pascalvoc 2007 or 2012 datasets using ssd_300_vgg.ckpt pre-trained model I am getting following error. </p>
  
  <pre><code>2017-08-25 20:03:03.001268: I tensorflow/core/common_runtime gpu/gpu_device.cc:1045] Creating TensorFlow device (/gpu:0) -&gt; (device: 0, name: Quadro M5000M, pci bus id: 0000:01:00.0)
  INFO:tensorflow:Error reported to Coordinator: &lt;type 'exceptions.ValueError'&gt;, Can't load save_path when it is None.
  Traceback (most recent call last):
    File "train_ssd_network.py", line 391, in &lt;module&gt;
      tf.app.run()
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/platform/app.py", line 48, in run
      _sys.exit(main(_sys.argv[:1] + flags_passthrough))
    File "train_ssd_network.py", line 387, in main
      sync_optimizer=None)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/contrib/slim/python/slim/learning.py", line 738, in train
      master, start_standard_services=False, config=session_config) as sess:
    File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
      return self.gen.next()
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/training/supervisor.py", line 965, in managed_session
      self.stop(close_summary_writer=close_summary_writer)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/training/supervisor.py", line 793, in stop
      stop_grace_period_secs=self._stop_grace_secs)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/training/coordinator.py", line 389, in join
      six.reraise(*self._exc_info_to_raise)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/training/supervisor.py", line 954, in managed_session
      start_standard_services=start_standard_services)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/training/supervisor.py", line 709, in prepare_or_wait_for_session
      init_feed_dict=self._init_feed_dict, init_fn=self._init_fn)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/training/session_manager.py", line 281, in prepare_session
      init_fn(sess)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/contrib/framework/python/ops/variables.py", line 660, in callback
      saver.restore(session, model_path)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/training/saver.py", line 1558, in restore
      raise ValueError("Can't load save_path when it is None.")
  ValueError: Can't load save_path when it is None.
  </code></pre>
  
  <p>I am using following script to fine-tune the model</p>
  
  <pre><code>DATASET_DIR=./tfrecords
  TRAIN_DIR=./logs/
  CHECKPOINT_PATH=./checkpoints/ssd_300_vgg.ckpt
  python train_ssd_network.py \
      --train_dir=${TRAIN_DIR} \
      --dataset_dir=${DATASET_DIR} \
      --dataset_name=pascalvoc_2012 \
      --dataset_split_name=train \
      --model_name=ssd_300_vgg \
      --checkpoint_path=${CHECKPOINT_PATH} \
      --save_summaries_secs=60 \
      --save_interval_secs=600 \
      --weight_decay=0.0005 \
      --optimizer=adam \
      --learning_rate=0.001 \
      --batch_size=10
  </code></pre>
  
  <p>The model ssd_300_vgg.ckpt is stored at the location ./checkpoints</p>
  
  <p>Please let me know if anyone has the solution. </p>