Created by: luotao1

• refine benchmark/.gitigone
• fix typo of argument learning_rate
• add argument --cpus for multi-thread cpus
• use a common function print_train_time to reduce lines
• mkdir -p logs to store all the logs
• fix --models=resnet50 to --models=resnet, since the file name resnet.py.
• fix the issue when running with --gpus > 1, duplicated with https://github.com/PaddlePaddle/Paddle/pull/11121#issue-191982770