from warmup_scheduler import GradualWarmupScheduler

scheduler_plateau = torch.optim.lr_scheduler.ReduceLROnPlateau(optimizer, patience=3, verbose=True)
scheduler_warmup = GradualWarmupScheduler(optimizer, multiplier=8, total_epoch=10, after_scheduler=scheduler_plateau)

for epoch in range(train_epoch):
    scheduler_warmup.step()     # 10 epoch warmup, after that schedule as scheduler_plateau
    ...