title: pytorch grad is None after .backward()
body: <p>I just installed <em>torch-1.0.0</em> on <em>Python 3.7.2</em> (macOS), and trying the <a href="https://pytorch.org/tutorials/beginner/blitz/autograd_tutorial.html#sphx-glr-beginner-blitz-autograd-tutorial-py" rel="noreferrer">tutorial</a>, but the following code:</p>
  
  <pre><code>import torch
  x = torch.ones(2, 2, requires_grad=True)
  y = x + 2
  z = y * y * 3
  out = z.mean()
  out.backward()
  print(out.grad)
  </code></pre>
  
  <p>prints <code>None</code> which is not what's expected.</p>
  
  <p>What's the problem?</p>