Learning backend cuda

Context

Suppport Learning with backend_cuda:

• For DataProvider, add a backend attribute to allow reading batch on supported backends
• Add operators to compute Accuracy with a graph: ArgMax, And, ReduceSum

Modified files

• DataProvider.hpp , DataProvider.cpp and pybind_DataProvider.cpp, add backend member to DataProvider;
• ArgMax.hpp , ArgMax.cpp and pybind_ArgMax.cpp, add ArgMax operator;
• And.hpp , And.cpp and pybind_And.cpp, add And operator;
• ReduceSum.hpp , ReduceSum.cpp and pybind_ReduceSum.cpp, add ReduceSum operator;

This MR also fixes: #155 (closed)

TODO

• Support backend_cuda in DataProvider
• Add ArgMax operator for Accuracy computation
• Add And operator for Accuracy computation
• Add ReduceSum operator for Accuracy computation