Add a sample op, `add_op`

* Convert `op` --> `operators`
* Remove AddType in OpProtoMaker, because type is part of registry.
* Rename CPU_OR_GPU --> DEVICE_TYPE in registry macro.

Default scope function

* Refine register methods, make Op can get rid of whole-archieve
* `USE_OP` before a op is used.
* Add unittest for add_op.

fix bug in dynload

Compile for armv8.

"fix init error"

go_binary: remove hardcoded library link path, add pserver client test

* Server recovery from checkpoint

Add go testing into cmake

Add unit test for OpKernel

Add GetEigenDevice To DeviceContext.

FIX: add -lrt for link

fix pybind compile question

OperatorWithKernel

* User can register OpKernel to its Ops. The OpKernelMap saved in
  OperatorWithKernel. Each Op which inherits OperatorWithKernel will
  use `OpKernel::Compute` instead of Run.