def diff(self, *args):
        """Call forward_mode; discard value, only keep the derivative."""
        arg_dicts = self._parse_args_forward_mode(*args)
        val, diff = self._forward_mode(*arg_dicts)
        return diff
def diff ( self , * args ) : arg_dicts = self . _parse_args_forward_mode ( * args ) val , diff = self . _forward_mode ( * arg_dicts ) return diff
Call forward_mode; discard value, only keep the derivative.