mxnet intalling and basiv knowledges

https://mxnet.incubator.apache.org/versions/master/install/index.html?platform=Linux&language=Python&processor=GPU
mxnet 模块化
Autograd API
Callback API
Contrib Package
Gluon API
Image API
IO API
KV Store API
Metric API
Module API
NDArray API
Optimization API
Profiler API
Run-Time Compilation API
Symbol API
Symbol in Pictures API
Tools

mxnet copyto and as_in_context辨析：
相同点：都是在设备间传递数据的，从gpu传递到cpu，或者从cpu传递到gpu
不同点：源和目标的context不一致，as_in_context不复制，而copyto总是会新建内存