def before_train_iter(self,
                          runner,
                          batch_idx: int,
                          data_batch: Optional[Sequence[dict]] = None) -> None:
        """fix lr of predictor by iter."""
        if self.adjust_by_epoch:
            return
        else:
            if self.fix_pred_lr:
                for param_group in runner.optim_wrapper.optimizer.param_groups:
                    if 'fix_lr' in param_group and param_group['fix_lr']:
                        param_group['lr'] = self.lr
fix lr of predictor by iter.
mmpretrain/engine/hooks/simsiam_hook.py