title: Why the grad is unavailable for the tensor in gpu
body: <pre><code>a = torch.nn.Parameter(torch.ones(5, 5))
  a = a.cuda()
  print(a.requires_grad)
  b = a
  b = b - 2
  print('a ', a)
  print('b ', b)
  loss = (b - 1).pow(2).sum()
  loss.backward()
  print(a.grad)
  print(b.grad)
  </code></pre>
  <p>After executing codes, the <code>a.grad</code> is <code>None</code> although <code>a.requires_grad</code> is <code>True</code>.
  But if the code <code>a = a.cuda()</code> is removed, <code>a.grad</code> is available after the loss backward.</p>