* wip simplify operator framework

* wip

* wip

* done test=develop

* clean test=develop

* fix test=develop

* fix deps test=develop

* fix cpu build test=develop

* fix tensorrt build test=develop

* fix tests test=develop

* fix test=develop

* fix cpu build test=develop

Fix code to support cpplint syntax check

test=develop

* wip

* add ref_by_trainer_id op

* ready to test

* fix ref inputs

* refine rpc_op_handle

* fix merge bug

Add dependency to send recv

Do not use ctor

* Reduce line of codes.
* We can use virtual function for Maker now.
* The implementation does not care what maker holds, it is easier to
refactor later.

* Fix CPPLint errors in operators

* Fix cast in softmax

* Fix softmax_mkldnn

* Fix send_recv_op_test

* Send_recv

* Fix softmax mkldnn

* add prefetch_op

* fix ci

* optimize code

* optimize code

* fix include

Fix sparse update memory error for distributed training

* dynamic send/recv selected rows

* update by comment

* fix by comment

* initialize batch barrier

* add some comments

* update

* fix batch barrier

* use sendvariable rpc interface to send batch barrier

* fix comment

* fix method

* fix by comment

* fix by comment

* Updating the cluster trainign doc

* Fixed comments

* Updating few comments in recv_op