title: Get UserWarning while I use tf.boolean_mask
body: <p>I got this UserWarning while using <code>tf.boolean_mask()</code>:</p>
  
  <pre><code>/usr/local/lib/python2.7/site-packages/tensorflow/python/ops/gradients_impl.py:93: UserWarning: Converting sparse IndexedSlices to a dense Tensor of unknown shape. This may consume a large amount of memory.
    "Converting sparse IndexedSlices to a dense Tensor of unknown shape. "
  </code></pre>
  
  <p>Once I remove it, the warning msg disappear. I find <a href="https://stackoverflow.com/a/35896823/5793660">this answer</a>, but I don't use <code>tf.gather()</code> and I am sure I pass the <code>tf.Variable</code> and <code>tf.placeholder</code> to <code>tf.nn.embedding_lookup()</code>. So how could I fix this problem?</p>
  
  <p>thanks for your help !</p>