add type in argparse

[tankeco]

环境

1.系统环境：ubuntu 2.MegEngine版本：1.2.0 3.python版本：3.6 4.模型名称：/

复现步骤

1. python3 train.py --arch resnet50 --batch-size 32 --learning-rate 0.025 --ngpus 8 --save .

请提供关键的代码片段便于追查问题

https://github.com/MegEngine/Models/blob/master/official/vision/classification/resnet/train.py#L58

The default type of argparse is str, so once batch_size, learning_rate or world_size is modified, it will cause an error