Get learning rate with polymomial 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 polymomial decay based on current iteration.

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

        Returns:
            float: Learning rate
        '''
        return self.init_lr * ((1.0 - iter * 1.0 / self.max_iter) ** self.power)