title: tf.train.latest_checkpoint returning none when passing checkpoint path
body: <p>When I am trying to load checkpoint after training ENet model for prediction using tf.train.latest_checkpoint(), it's returning "None" though I am passing the correct checkpoint path.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>     image_dir = './dataset/test/'
       images_list = sorted([os.path.join(image_dir, file) for file in 
       os.listdir(image_dir) if file.endswith('.png')])
  
       checkpoint_dir = "./checkpoint_mk"
       listi = os.listdir(checkpoint_dir)
       print(listi)
       checkpoint = tf.train.latest_checkpoint("./log/original/check")
       print(checkpoint,'--------------------------------------- 
       ++++++++++++++++++++++++++++++++++++++++++++++++++++')
  </code></pre>
  
  <p>It returns None.</p>
  
  <p>I am passing absolute path of checkpoint as they store in some other Dir.</p>
  
  <p>Here are my checkpoint folder.</p>
  
  <p><a href="https://i.stack.imgur.com/P1NFt.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/P1NFt.png" alt="enter image description here"></a></p>
  
  <p>EDIT ---------------</p>
  
  <p>model_checkpoint_path: "model.ckpt-400"</p>
  
  <p>all_model_checkpoint_paths: "model.ckpt-0"</p>
  
  <p>all_model_checkpoint_paths: "model.ckpt-400"</p>