Design Doc: Distributed Training Architecture

add OMP SGD to speedup with CPUs

Feature/remove int pair

* fix nv_library

* fix symbol in gpu_info.h

First half translation of operator definition.

Sync computation when Python invoke `run`

Simplify GetAttrType code

Stabilize prelu gradient check

lstm unit

enable VGG with MKLDNN layers

Add gemm with stride arguments

add namespace and reduce warning

Update annotation in layers.py and update FAQ.

Add multiplex operator

implement framework of seq_pool_op and its unitest

adding doc for comparing cluster bootstrapping tools

Change LoDTensorDesc::dims from int32 to int64