In Tensorflow, what is the difference between a tensor that has a type ending in _ref and a tensor that does not? : <p>The docs say:</p>

<blockquote>
  <p>In addition, variants of these types with the _ref suffix are defined
  for reference-typed tensors.</p>
</blockquote>

<p>What exactly does this mean? What are reference-typed tensors and how do they differ from standard ones?</p>
