    def step_update(self, num_updates):
        """Update the learning rate after each update."""
        cycle = math.floor(num_updates / (2 * self.stepsize))

        lr_shrink = self.lr_shrink**cycle
        max_lr = self.max_lr * lr_shrink
        if self.shrink_min:
            min_lr = self.min_lr * lr_shrink
        else:
            min_lr = self.min_lr

        x = abs(num_updates / self.stepsize - 2 * (cycle + 1) + 1)
        self.lr = min_lr + (max_lr - min_lr) * max(0, (1 - x))

        self.optimizer.set_lr(self.lr)
        return self.lr
Update the learning rate after each update.
fairseq/optim/lr_scheduler/triangular_lr_scheduler.py