enable_grad
class torch.enable_grad
Context-manager that enables gradient calculation.
Enables gradient calculation, if it has been disabled via "no_grad"
   or "set_grad_enabled".
This context manager is thread local; it will not affect
   computation in other threads.
Also functions as a decorator. (Make sure to instantiate with
   parenthesis.)
Note:
 enable_grad is one of several mechanisms that can enable or
 disable gradients locally see Locally disabling gradient
 computation for more information on how they compare.

Note:
 This API does not apply to forward-mode AD.

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