So for example, in the first 150 iterations I want full precision training, and in the later 150 iterations I want to switch back to mixed precision, and switching like this so on and so forth. The scaler is set before the training begin, i.e. scaler = torch.cuda.amp.GradScaler(enabled=use_amp), then do you mean that during the training, say after 150 iterations, I need to re-initialize this scaler and change the enabled to true or false?