support training with c++

[jacquesqiao]

Background

Some system need to train neural network using C++ API of fluid, so fluid need to provide the interface and a demo to do this.

TODO

• 1. User use Python to describe the network then save the ProgramDesc to disk.
• 2. Add a CPP interface to load the ProgramDesc from disk.
• 3. use CPP Executor to run the ProgramDesc
• 4. Add document

[jacquesqiao]

第一阶段已完成