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.