Add float16 support to dropout operator

Fix bugs in c++ readers

add math_function to softmax's dep list

move distributed lookup table design to fluid/dist_train

Repair deadlink of fluid doc

repair link of "Executor"

Fix compilation for gcc5.4

* Support copy in Fluid channels

* Address PR review comments

Enhance LoDResetOp and add python wrapper

Disable the link flags on Mac.

Enable P2P memory copy

On k40 with 4 devices, time reduces from ~4.0 to ~3.8+, should be
more obvious on better hardware

move two images

Limit the symbol table of fluid shared library.

Add distributed lookup table design

Remove unnecessary clone of program in C++ Executor.Run

Add float16 support for pool 2d operator

The error is:

paddle/fluid/operators/math/concat.cc:47:72: error: invalid initialization of non-const reference of type 'paddle::platform::CPUPlace&' from an rvalue of type 'paddle::platform::CPUPlace'
   auto& cpu_place = boost::get<platform::CPUPlace>(context.GetPlace());

Should not use reference for cpu_place.

Add float16 support for cast op