src/transformers/trainer_callback.py/CallbackHandler/add_callback
class CallbackHandler:    def add_callback(self, callback):
        cb = callback() if isinstance(callback, type) else callback
        cb_class = callback if isinstance(callback, type) else callback.__class__
        if cb_class in [c.__class__ for c in self.callbacks]:
            logger.warning(
                f"You are adding a {cb_class} to the callbacks of this Trainer, but there is already one. The current"
                + "list of callbacks is\n:"
                + self.callback_list
            )
        self.callbacks.append(cb)