The documentation page MAIN_CLASSES/コールバック doesn’t exist in v4.40.2, but exists on the main version. Click <a href="/docs/transformers/main/ja/main_classes/%E3%82%B3%E3%83%BC%E3%83%AB%E3%83%90%E3%83%83%E3%82%AF">here</a> to redirect to the main version of the documentation.