def on_epoch_end(self, runner: "IRunner") -> None:
        """Check if validation metric should be dropped for current epoch.

        Args:
            runner: current runner
        """
        if self.valid_loader_key not in runner.loaders:
            runner.epoch_metrics[self.valid_loader_key] = {
                self.valid_metric_key: float("+inf")
                if self.minimize_metric
                else float("-inf")
            }
Check if validation metric should be dropped for current epoch.

        Args:
            runner: current runner
        
catalyst/callbacks/periodic_loader.py