Created by: NHZlX

Add load the model from memory interface for cxx_api.

 std::ifstream model_f(FLAGS_model_dir + "/__model__");  
 std::stringstream buffer;
 buffer << model_f.rdbuf();
 std::string model_c(buffer.str()); 
 ...
 cfg.set_model_buffer(model_c.data(), model_c.size(), params_c.data(), params_c.size());