def on_loader_start(self, runner: "IRunner") -> None:
        """
        Loader start hook: initiliaze storages for the loaders.

        Args:
            runner: current runner
        """
        super().on_loader_start(runner)
        if runner.loader_key == self._train_loader:
            self.storage.reset(
                num_samples=runner.loader_sample_len, num_batches=runner.loader_batch_len
            )
        if runner.loader_key in self._valid_loaders:
            assert self.model is not None, "The train loader has to be processed first!"

        Loader start hook: initiliaze storages for the loaders.

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