[WIP]: feature/parallel_do

Fix adagrad op by removing broadcast of Eigen.

Parallel do

* "add c++ side kernel selection"

* "add multiple kernel op test"

* "kernel selection only support cudnn"

* "better formatter"

* "small fix with UseCPU"

* "depends on change interface Get(Place, Library)"

* "fix CI"

* "fix python cudnn test"

* "leave the register cudnn op to another PR"

* "fix CI"

* "use all kernel by default"

* "fix CI"

Add the concurrent programming design doc

* send_recv variable

* delete unused logs

* fix ci failed

* update

* resize tensor before tensor copy

* add selectedrows unit test

* check rows

Add a simple C++ inference example for fluid

Correctly handle lod information of image operators

* Add squared error layers doc

* Add doc for gru_unit

* Remove cdot which isn't supported

* Update layers.rst

* Update layers.rst (minor)

Async to drop kid

Default use one thread in fluid

Update the version of openblas for Android and iOS

Add flag use_mkl_packed

* "remove cudnndevicecontext"

* "remove unused init code"

* "fix hash functions"

Expose some activations

* "fix typo"

* "fix based on comments"

* "follow gogle style"

* "fix based on comemnts"

Tiny enhance of while_op

Doc fix and enhancement for lstm_unit python wrapper.

* "fix frigled test gradient of rnn"

* "fix based on comments"

refine pybind when *_op.cc contains several operators