Paddle cmake build environment is not a clean out-of-source build.

[wanglei828]

1. In paddle repo, python code comes from two parts, original python files in python directory and generated python files from proto. Current build system will put all the generated files into source directory not build directory, mixed with code repo.
2. When running unittest using ctest command, all the python related tests will be run from source code repo, which means pyc files will be added to the source code repo. Also intermediate model files will be also put into source code.
   As a clean out-of-source cmake build, when you create a build directory and running make and test, all the files will be generated into build directory. Source code repo will not be changed.