[WIP] C-API <--> Python API for paddle.framework

[reyoung]

• Add new C-API for refactored Paddle, in paddle/c
• Expose that C-API to paddle.v2.framework.c.Variable and
  paddle.v2.framework.c.Scope
  • Use Cython to warp them and give a native Python API.
• Add Unit tests.
  • See python/paddle/v2/framework/tests/test_scope.py for usage.

[wangkuiyi]

Do we really need the C API?

I think that for the MVP, all we need is a Python API. And the Go API doesn't need to have as many features as the Python API #2750 (comment). Is this correct? Or, what are supposed to be exposed via the Go API?

[wangkuiyi]

这个文件是干啥呢？

[wangkuiyi]

If we use Boost::Python, all the following files in this PR

• scope.h
• scope.cc
• private.h
• error.h
• dllexports.h

would be replaced by the following few-line C++ code:

#include <boost/python.hpp>

BOOST_PYTHON_MODULE(pypaddle) {
    boost::python::class_<paddle::framework::Scope>("Scope")
        .def("create_var", &Scope::CreateVar)
        .def("create_scope", &Scope::CreateScope);
}

Please re-think about this necessity of create a C-API.

[gangliao]

Pybind11 is also an excellent candidate
https://github.com/pybind/pybind11
https://media.readthedocs.org/pdf/pybind11/master/pybind11.pdf

Caffe2 chose pybind11 to expose c++11 directly.

[reyoung]

Whether uses C-API depends on is there any other languages need invoke Paddle C++ Core or not.

I am not sure only Python API is enough or not. At least there are several needs for us to give a C-API.

• Android/IOS API. Give a Java/Objective-C wrapper for Paddle.
• Go API. Needs for our Parameter Server.

Also, I think pybind11 is better than boost::Python because Paddle is in C++ 11 .

But if we have a C-API for Paddle, wrap that C-API to Python is extremely easy by Cython.

cdef extern from "math.h":
    double sin(double x)

[reyoung]

This PR is just an exploring PR for how to expose Paddle C++ core by C-API & Cython. I will give another PR by using pybind11.