* 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

* fix bug of fuse optimize ops

* replace ThreadedExecutor with FastThreadedExecutor
test=develop

* Fix Travise CI
test=develop

* Test FastThreadedSSAGraphExecutor
test=develop

* refine parallel_ssa_graph_executor.cc
test=develop

* Add an option to enable the cache of expected kernel in train phase.
test=develop

* Change the default value of cache_expected_kernel to true.

test=develop

* bug-fix, test=develop

* tweak code, test=develop

* test fuse all_reduce

* Try to enable the runtime_context_cache pass in train phase.

* Put the append of runtime_context_cache pass ahead of multi_dev passes.
test=develop

* expose fuse broadcast ops

* Fix the bug of the fast threaded executor. I

* fix the bug of reusing different types of variables in memory_optimize_pass, test=develop

* disable SELECTED_ROWS AND LOD_TENSOR_ARRAY reusage, test=develop

test=develop

* fix compiled
test=develop

* follow comments test=develop

* fix comments of 16410, test=develop

* modify inplace_op_inference_test according to pass interface change, test=develop

* fuse optimizer

* fix threaded executor bug
test=develop

* change the order of class member
test=develop

* Fix Travis CI
test=develop

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

test=develop

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

test=develop

* test fix fetch bar place for ce

* fix ps mode dist train in develop test=develop

* fix style check test=develop

* update test=develop

test=develop