mxnet变量和模型的加载及保存
https://littletomatodonkey.github.io/2018/05/23/2018-05-23-mxnet%E5%9F%BA%E6%9C%AC%E4%BD%BF%E7%94%A8%E4%BB%A5%E5%8F%8A%E6%A8%A1%E5%9E%8B%E5%8A%A0%E8%BD%BD%E4%B8%8E%E4%BF%9D%E5%AD%98/
https://littletomatodonkey.github.io/2018/05/23/2018-05-23-mxnet%E5%9F%BA%E6%9C%AC%E4%BD%BF%E7%94%A8%E4%BB%A5%E5%8F%8A%E6%A8%A1%E5%9E%8B%E5%8A%A0%E8%BD%BD%E4%B8%8E%E4%BF%9D%E5%AD%98/