title: requires_grad relation to leaf nodes
body: <p>From the <a href="https://media.readthedocs.org/pdf/pytorch-zh/latest/pytorch-zh.pdf://" rel="noreferrer">docs</a>:</p>
  
  <blockquote>
    <p>requires_grad – Boolean indicating whether the Variable has been
    created by a subgraph containing any Variable, that requires it.  Can
    be changed only on leaf Variables</p>
  </blockquote>
  
  <ol>
  <li>What does it mean by leaf nodes here? Are leaf nodes only the input nodes?</li>
  <li>If it can be only changed at the leaf nodes, how can I freeze layers then?</li>
  </ol>