1. Add attr 'average' into ParamAttr.
2. Make 'params_grads' optional for AverageModel.
3. Add option 'average_mean' and 'average_variance' for batch_normal.

* add prefetch_op

* fix ci

* optimize code

* optimize code

* fix include

Cpp parallel executor

* Fix Issue 9388

* Fix typos

Revert "make append activation in place by default (#9417)"

This reverts commit ce16400d.

Update c++ readers doc

* "add functors"

* "remove old code"

* "fix"

* "fix ci"

* "add details"

* "fix ci"

* "fix ci"

* "fix ci"

* "fix ci"

* "remove unused code"

fix dist train selected rows height missing

Add cos and sin

* Add channel design document

* Update channel send/recv state diagram

added missing *.pb.h *.pb.cc generation to fix distribute build issue

remove vars when remove ops

In place batch_norm

fix submit_local's paddle pip name issue