title: How to solve issue : grad_fn = None
body: <p>Could anyone advise why <code>grad_fn = None</code> for this <a href="https://github.com/promach/gdas/blob/main/gdas.py#L447" rel="nofollow noreferrer">pytorch coding</a> ?</p>
  <p><a href="https://i.stack.imgur.com/xcmY7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xcmY7.png" alt="grad_fn = None" /></a></p>