polish parallel dygraph code#17164

Merged

chengduoZH merged 8 commits intoPaddlePaddle:developfrom

Yancey0623:parallel_dygraph_unittest

May 17, 2019

Commits on Apr 29, 2019

add var grad hook test=develop
Yancey1989
committed
polish code test=develop
Yancey1989
committed
polish code test=develop
Yancey1989
committed

Commits on Apr 30, 2019

fix unittest on cpu test=develop
Yancey1989
committed

Commits on May 5, 2019

fix win build test=develop
Yancey1989
committed
fix win build test=develop
Yancey1989
committed

Commits on May 6, 2019

fix loss scale test=develop
Yancey1989
committed
polish code test=develop
Yancey1989
committed