Implement backward function of Add operator

Context

Add backward kernel for Add.

partially fix #30 (closed)

include/aidge/backend/cpu/operator/AddImpl.hpp

@@ -25,7 +25,19 @@
 namespace Aidge {
 // Operator implementation entry point for the backend
 using AddImpl_cpu = OperatorImpl_cpu<Add_Op,
-    void(std::vector<std::size_t>, std::vector<std::size_t>, const std::vector<std::size_t>&, const void*, const void*, void*)>;
+    void(std::vector<std::size_t>, std::vector<std::size_t>, const std::vector<std::size_t>&, const void*, const void*, void*),
+    void(const std::size_t, 
+         const std::size_t, 
+         const std::size_t, 
+         const std::vector<std::size_t>&, 
+         const std::vector<std::size_t>&, 
+         const std::vector<std::size_t>&, 
+         const void*, 
+         const void*, 
+         const void*, 
+         void*, 
+         void*)
+>;
 
 // Implementation entry point registration to Operator
 REGISTRAR(Add_Op, "cpu", Aidge::AddImpl_cpu::create);