tensors.
Example::
      >>> x = torch.tensor([1.], requires_grad=True)
      >>> with torch.no_grad():
      ...     y = x * 2
      >>> y.requires_grad
      False
      >>> @torch.no_grad()
      ... def doubler(x):
      ...     return x * 2
      >>> z = doubler(x)
      >>> z.requires_grad
      False