* support control flow

* supoort sync_parameters_buffers

* fix the bug of sparse embedding

* fix the bug of reducer in embedding

* add comment

* 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