Get learning rate with exponential decay based on current iteration.

        Args:
            iter (int): Current iteration (starting with 0).

        Returns:
            float: Learning rate

def get_learning_rate(self, iter):
        '''
        Get learning rate with exponential decay based on current iteration.

        Args:
            iter (int): Current iteration (starting with 0).

        Returns:
            float: Learning rate
        '''
        lr = self.init_lr
        for iter_step in self.iter_steps:
            if iter >= iter_step:
                lr *= self.gamma
        return lr