test=develop

* random failed. rerun ci. test=develop

* windows failed. rerun ci. test=develop

test=develop

* add more log for dist test for ci test=develop

* increase deadline test=develop

test=develop

test=develop

Revert "Merge pull request #14798 from PaddlePaddle/revert-14786-revert-14782-revert-14398-imperative"

This reverts commit b1d3a1c8, reversing
changes made to f1fb64b1.

* enable test_gradient_clip in mac
test=develop

* refine test_gradient_clip
test=develop

* fix io doc
test=develop

* refine data_feeder doc
test=develop

* add nccl2 dist tests test=develop

* fix dist_base test=develop

* fix tests test=develop

* fix test on mac test=develop

* Refine conv_fusion_op for identity activation.
* Fix unit testing.
test=develop

test=develop

test=develop

* test dist ut fixes test=develop

* fix cmake

* for test

* expose square
test=develop

* fix activation
test=develop

* Add square API
test=develop

* add necessary op

* code refine

* fix API.spec
test=develop

* fix unit test
test=develop

* add unit test sparse_grad_clip
test=develop

* fix API.spec
test=develop

* remove mac test for test_gradient_clip
test=develop

* remove selectedrows_mul_tensor
test=develop

test=develop

test=develop

test=develop

* wip multi process multi gpu dist training

* workable for p2p

* update test=develop

* change back env name test=develop

* fix alloc init

* fix cpu build test=devlop

* fix mac tests test=develop

* refine code

* refine test=develop

* layer fixes test=develop

* follow update test=develop

* fix api format and examples

test=develop

* Update executor.py

test=develop

* Update nn.py

* Update nn.py

test=develop

* Update nn.py

test=develop

test=develop

test=develop

test=develop

test=develop

test=develop

test=develop