Feature/change assign op

Add comments and polish code

force clean up process when no_clean_up is set true

* Fix CPPLint errors

* Fix CPPLint errors in sequence2batch

* Fix compilation

* Fix LSTM op and GRU op

* Fix LSTMP op

* Fix more cpplint errors in operators/math

* Address Code review feedback

fix nccl2 version in manylinux devel docker image

Clean memcpy async

fix mac compile errors

Fix unittests

Add init interface for customize devices.

* modify roi pool with lod and expose ROI Pooling into Python API

* make lod code brief

* make doc more clearly

* make doc more clearly

add missing grad_to_block_id

* Some changes for Adam profiling

* Adam optimization: initial Eigen optimization

* Eigen Adam: flavour of adam can be chosen

* Eigen Adam used for CPU by default. Plain Adam used for GPU

* Eigen Adam: missing call to the Eigen functor added

* Eigen Adam: revert changes in benchmarks

* Eigen Adam: typo corrected

* add float16 transpiler

* fix feed fetch target names mismatch

* fix cast op input change issue

* fix program desc flush error

* fix inconsistent var names in block desc bug

* code clean up

* add float16 infernce C++ example and fix prune bug

* Fix CPPLint issue in test_engine

* Fix CPPLint errors in operators/math

* Fix compilation

listen_and_serv_op support async update

* Fix more CPPLint issues

* Fix more CPPLint issues

* Fix more CPPLint issues

* Fix CPPLint issues in operators/math and operators/reader

Change paddle.v2.dataset to paddle.dataset for V2 docs

* Update stacked_dynamic_lstm.py

* Update machine_translation.py

* Update mnist.py

* Update resnet.py

* Update vgg.py