Add version dropdown to the layout html (#873)

Summary:
Pull Request resolved: #873

Add the version dropdown to the layout html

Reviewed By: dbort

Differential Revision: D50238907

fbshipit-source-id: 6ffdacc8149d387275344973dd25643abc64c1d0

Author: svekars

docs/source/_templates/layout.html

@@ -19,7 +19,10 @@
 
 
 {% block sidebartitle %}
-{% include "searchbox.html" %}
+    <div class="version">
+      <a href='https://pytorch.org/executorch/versions.html'>{{ version }} &#x25BC</a>
+    </div>
+    {% include "searchbox.html" %}
 {% endblock %}
 
 {%- block content %}