* add doc for memory_optimize, test=develop

* update doc, test=develop

* doc update, test=develop

* Speed affine_channel_op unit testing
* Add check in tensor_py
* Fix ONLY_CPU Compiling

* 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

test=develop

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 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

test=develop

add NCCLParallelContext for parallel dygraph

test=develop

* expose fuse broadcast ops

test=develop

test=develop

test=develop

test=develop

test=develop

test=develop

* fuse optimizer

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

add include headers to fix travis-ci
test=develop

* extend the slice function for python
test=develop

test=develop

test=develop