* add reducer

* refine envent for memorycopy

* add concat&split for allreduce

* apply concat & split for fuse tensor

* fix nccl dep

* fix the untest, compile problem and ddp initialize problem

* fix untest for mac & add some comments & solve the repeated param in sublayers

* fix untest for windows & fix document

* remove slelected rows all reduce over height check

* polish unittest

* support selectedrows allreduce in multi-cards dygraph, test=develop

* remove useless import modules in unittests, test=develop

* add nccl cmake to get nccl version, test=develop

* add if-condition to compiled correctly, test=develop

* add detail version parseing for old nccl, test=develop

* polish camke details, test=develop

* fix remove test cmake error, test=develop

* fix cmake condition, test=develop

* change unittest camke list, test=develop

* fix unittest cmake rule, test=develop, test=framep0