when while_grad, it writes multiple @EMPTY@ with no VarDesc.

* add rw lock test

* optimize read_write and wirte_read test

Optimize selected rows for dist lookup table with rwlock 

* Add write after write

* follow comment

Reduce one level of inheritence.

Fix pb_protobuf2 FromString problem

Fix str and bytes problem
Fix sorted problem
Fix math problem
Fix CI problem

* fix the order of sum

* add doc

* check whether need to copy

* follow comments

* "cherry picked cpp tests"

* "cherry picked"

* "cherry picked tests"

* "merge develop branch"

* "add gradient register"

* "make some enhance"

* "better format"

* "fix typo"

* "fix reuse"

* "fix get expected kernel"

* "change the mkldnn code"

* "fix mkldnn"

* "fix mkldnn failed test"

* "add comment"

Add a few more RecordEvent.
Cleanup