Add distributed lookup table design

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

Add float16 support for pool 2d operator

Add float16 support for cast op

add gserver to capi dep

rm unused private field in profiler

fix regularizer when gradient is None

Update cpp reader doc

delete unused images

Add float16 support for cudnn conv2d

remove legacy design image

Fix a program copy regression.

fix AttributeError: 'module' object has no attribute 'framework_pb2'

add math_function to selected_rows_functor dependency list

change the dir of docs

Single device on se-resnet reduce from 0.56 to 0.50

* New test for testing channels / concurrency

* Formatting fix

* update en doc for Distributed Training

* typo fix

* Fix a critical bug in softmax_with_cross_entropy_op, which will lead to the wrong gradients.

* Enhance unit testing.