def exp_lr_scheduler(optimizer, epoch, init_lr=0.001, lr_decay_epoch=70):
    """Decay learning rate by a factor of 0.1 every lr_decay_epoch epochs."""

    for param_group in optimizer.param_groups:
        decaied = param_group['lr'] * 0.5
        param_group['lr'] = init_lr if decaied <= 2e-7 else decaied

    print("decay lr")
    return optimizer