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

* add device_context

* add unittest for device_context

* transfer to use function paddle::platform::throw_on_error

* fix cuda build error

* using dynload functions

* follow comments

Basically from caffe2::logging.h, but only expose `PADDLE_ENFORCE`
interface.

__must_check is a macro mark of function return value. It let developer
must check the return value is legal or not.