Design doc: save model in cluster training.

ENH: support command `make target_name`

fix the bug of copy master directory.

NNPACKConvFunction

FIX: Replace static libs check via system warning

Add the use_nnpack parameter in ExpandConvLayer, so that the convolution calculation can be switched to the NNPACK function.

Design doc for go package management

FIX: Always build protobuf from source

ENH: Remove deprecated python build

Improve Go cmake, add go/master/c into cmake include path..

FIX: merge objects to static lib

fix bugs

add more choice for eigen downloading

enable users to set intial memory states for lstm/gru group.

Add pb_cc_library in generic.cmake

basic scope implementation #2629

* scope-impl

* refine GetOrCreateVariable

* refine code of scope with style check

* refine code of operator

* clang format

* move code to scope.h, remove scope.cc

* add Create for scope

* merge CreateVar and GetOrCreateVar

* do not use default argument in Scope

* update design doc