def on_loader_start(self, runner: "IRunner"):
        """Loader start hook. Updates scheduler statistics.

        Args:
            runner: current runner
        """
        if runner.is_train_loader:
            lr_step = self.final_lr / self.init_lr
            self.num_steps = self.num_steps or runner.loader_batch_len
            self.multiplier = lr_step ** (1 / self.num_steps)
            self.lr_step = (self.final_lr - self.init_lr) / self.num_steps

        super().on_loader_start(runner=runner)
Loader start hook. Updates scheduler statistics.

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