mxnet operator

如何构建运行时参数，包括context，opkernel等。

https://github.com/dmlc/mxnet/blob/master/src/c_api/c_api_ndarray.cc#L366

op_executor

真正执行Op的上下文。

AttachOpExecs

将kernel attach到graph上以便可以运行。实现在这里

运行时寻找kernel

https://github.com/dmlc/mxnet/blob/master/src/executor/attach_op_execs_pass.cc#L163

mxnet operator 注册机制详解

https://github.com/dmlc/mxnet/issues/3248