fix: Add operator backward warnings

Context

Fix warnings introduced in !137 (merged), where some parameters of the Add backward kernels were unused.

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

@@ -33,8 +33,6 @@ using AddImpl_cpu = OperatorImpl_cpu<Add_Op,
          const std::vector<std::size_t>&, 
          const std::vector<std::size_t>&, 
          const void*, 
-         const void*, 
-         const void*, 
          void*, 
          void*)
 >;