How to debug in VSCode when program exec to os.system(cmd)

I want to debug in vscode for following codes. But it didnot run as expected!

def main():
    args = parse_args()
    if args.mode == "single":
        train_cmd = "python lib/train/run_training.py --script %s --config %s --save_dir %s --use_lmdb %d " \
                    "--script_prv %s --config_prv %s --distill %d --script_teacher %s --config_teacher %s --use_wandb %d"\
                    % (args.script, args.config, args.save_dir, args.use_lmdb, args.script_prv, args.config_prv,
                       args.distill, args.script_teacher, args.config_teacher, args.use_wandb)
    elif args.mode == "multiple":
        train_cmd = "python -m torch.distributed.launch --nproc_per_node %d --master_port %d lib/train/run_training.py " \
                    "--script %s --config %s --save_dir %s --use_lmdb %d --script_prv %s --config_prv %s --use_wandb %d " \
                    "--distill %d --script_teacher %s --config_teacher %s" \
                    % (args.nproc_per_node, random.randint(10000, 50000), args.script, args.config, args.save_dir, args.use_lmdb, args.script_prv, args.config_prv, args.use_wandb,
                       args.distill, args.script_teacher, args.config_teacher)
    elif args.mode == "multi_node":
        train_cmd = "python -m torch.distributed.launch --nproc_per_node %d --master_addr %s --master_port %d --nnodes %d --node_rank %d lib/train/run_training.py " \
                    "--script %s --config %s --save_dir %s --use_lmdb %d --script_prv %s --config_prv %s --use_wandb %d " \
                    "--distill %d --script_teacher %s --config_teacher %s" \
                    % (args.nproc_per_node, args.ip, args.port, args.world_size, args.rank, args.script, args.config, args.save_dir, args.use_lmdb, args.script_prv, args.config_prv, args.use_wandb,
                       args.distill, args.script_teacher, args.config_teacher)
    else:
        raise ValueError("mode should be 'single' or 'multiple'.")
    os.system(train_cmd)  # here

When step debuger executes to os.system(train_cmd), vscode debuger cannot run as expected. Instead, it run the train_cmd in debug console. Although program can run normally, I cannot debug. Who can help me, thanks a lot!!!