Add gtag() helper function used by star-rating

Originally proposed this change in the `pytorch/pytorch_sphinx_theme`
repo as PR#188, but SveKars suggested we try it out in
`pytorch/tutorials` which does not have as large a scope.

Author: carljparker

_templates/layout.html

@@ -17,7 +17,11 @@
 {{ super() }}
 <script>
 
-  
+<!--
+  Helper function to make it easier to call dataLayer.push()
+-->
+function gtag(){window.dataLayer.push(arguments);}
+
 //add microsoft link
 
 if(window.location.href.indexOf("/beginner/basics/")!= -1)