add spp（Spatial pyramid pooling ） op 

fix simple_gru2 doc

* "add DeviceContextPool"

* "add devicecontextpool in pybind"

* "add comments in python side "

* "fix static link error"

* "fix CI error"

* "add executor.py"

* "fix CI error"

* "add with gpu macro"

* "remove comment out codes"

* "add TODO items"

* "update init devices"

* add more place_test and rename Cudnn to CUDNN

* fix ci

Optimizer is not a part of fluid. The `paddle::platform`
was missing.

use Intel OpenMP to speedup seq2batch when WITH_MKL

add fluid python api in chinese website

Reduce memory usage in conv layer and RoI layer for mobile inference.

Enable reshape_op to support dimension inference

add MKL Packed design doc for RNN optimazation

Add refer code to get started ~

fix pipe reader on multi passes

Add ChunkEvaluator for Multi-batches

Fix typo in cross-compiling documentation of iOS and add it to the index

add ios cross compiling doc in en

Multigpu Feature