keras/src/optimizers/base_optimizer.py/BaseOptimizer/_get_current_learning_rate
class BaseOptimizer:    def _get_current_learning_rate(self):
        if isinstance(
            self._learning_rate, learning_rate_schedule.LearningRateSchedule
        ):
            return self._learning_rate(self.iterations)
        elif callable(self._learning_rate):
            return self._learning_rate()
        return self._learning_rate