def METHOD_NAME(self, data):
    """Preprocess the data fetched from the inner_dataset.
    If the preprocessor is None, the original data will be returned, else the preprocessor will be called.
    User can override this method to implement custom logics.
    Args:
        data: The data fetched from the dataset.
    Returns: The processed data.
    """
    return self.preprocessor(
        data) if self.preprocessor is not None else data