Following the design
* https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/python_api.md

Just written `Program`, `Block` and unittest of program.

Rename platform::GetDeviceCount into platform::GetCUDADeviceCount

Design Doc: Session

Update lod_tensor.md

Change `PADDLE_ONLY_CPU` to `PADDLE_WITH_GPU`

By shell command

```bash
sed -i 's#ifdef PADDLE_ONLY_CPU#ifndef PADDLE_WITH_GPU#g' `find ./paddle/ -name '*.h' -o -name '*.cc' -o -name '*.cpp' -o -name '*.c' -o -name '*.cu'`
sed -i 's#ifndef PADDLE_ONLY_CPU#ifdef PADDLE_WITH_GPU#g' `find ./paddle/ -name '*.h' -o -name '*.cc' -o -name '*.cpp' -o -name '*.c' -o -name '*.cu'`
```

Bug fix

Add helper function in GradOpDescMakerBase. Make it easier to use.

add generic add operator

minor language fixes

gather, scatter with gpu support, passed python test

* Changing learning rate from attribute to input(float)
* Removing obsolete code

Feature/simplify sum op

Fix MacOS compile error