add 1.7.0 to the version selector

Author: mattip

docs/versions.html

@@ -20,6 +20,9 @@ <h1> PyTorch Documentation </h1>
         <li class="toctree-l1">
           <a class="reference internal" href="master/">master (unstable)</a>
         </li>
+        <li class="toctree-l1">
+          <a class="reference internal" href="1.7.0/">v1.7.0 (pre-release)</a>
+        </li>
         <li class="toctree-l1">
           <a class="reference internal" href="stable/">v1.6.0 (stable release)</a>
         </li>