title: What's the differences between tf.GraphKeys.TRAINABLE_VARIABLES and tf.GraphKeys.UPDATE_OPS in tensorflow?
body: <p>Here is <a href="https://www.tensorflow.org/api_docs/python/tf/GraphKeys" rel="noreferrer">doc</a> of tf.GraphKeys in tensorflow, such as <code>TRAINABLE_VARIABLES</code>: the subset of Variable objects that will be trained by an optimizer. </p>
  
  <p>And i know <code>tf.get_collection()</code>, which can find some tensor that you want.</p>
  
  <p>When use <code>tensorflow.contrib.layers.batch_norm()</code>, the parameter <code>updates_collections</code> default value is <code>GraphKeys.UPDATE_OPS</code>. </p>
  
  <p>How can we understand those collections, and difference in them.</p>
  
  <p>Besides, we can find more in ops.py.</p>