set_grad_enabled
class torch.set_grad_enabled(mode)
Context-manager that sets gradient calculation on or off.
"set_grad_enabled" will enable or disable grads based on its
   argument "mode". It can be used as a context-manager or as a
   function.
This context manager is thread local; it will not affect
   computation in other threads.
Parameters:
      mode (bool) -- Flag whether to enable grad ("True"), or
      disable ("False"). This can be used to conditionally enable
      gradients.
Note:
 set_grad_enabled 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)
      >>> is_train = False
      >>> with torch.set_grad_enabled(is_train):
      ...     y = x * 2
      >>> y.requires_grad
      False