* speedup gc and inplace softmax_with_cross_entropy_grad
test=develop

* refine models gpu mem
Merge skip vars and warning messages of mem opt
remove relu mem opt
test=develop

* follow comments
test=develop

* modify the interface of Pass::Allay
test=develop

* Polish code
test=develop

* Fix Travis CI
test=develop

* fix Pass::Apply interface
test=develop

* Fix Travis CI
test=develop

* fix cdn issue, test=develop

* fix memory optimize bugs, test=develop

* fix memory optimize bugs, test=develop

* remove add/sub_2 op, test=develop

* disable memory_optimize by default, test=develop

* disable inplace activation in python, test=develop

* fix unittests, test=develop

* fix unittests, test=develop

* bug-fix, test=develop