def add_listener(self, listener: "Tok2VecListener", component_name: str) -> None:
        """Add a listener for a downstream component. Usually internals."""
        self.listener_map.setdefault(component_name, [])
        if listener not in self.listener_map[component_name]:
            self.listener_map[component_name].append(listener)
Add a listener for a downstream component. Usually internals.
spacy/pipeline/tok2vec.py