Add backward pass CPU implementation for Mul operator

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

@@ -25,11 +25,25 @@ namespace Aidge {
 
 // compute kernel registry for forward and backward
 class MulImplForward_cpu
-    : public Registrable<MulImplForward_cpu, std::tuple<DataType, DataType, DataType>, void(const std::vector<std::size_t>&, const std::vector<std::size_t>&, const std::vector<std::size_t>&, const void*, const void*,void*)> {
-};
+    : public Registrable<MulImplForward_cpu, std::tuple<DataType, DataType, DataType>, void(const std::vector<std::size_t>&, 
+                                                                                            const std::vector<std::size_t>&, 
+                                                                                            const std::vector<std::size_t>&, 
+                                                                                            const void*, 
+                                                                                            const void*,
+                                                                                            void*)> {};
+
 class MulImplBackward_cpu
-    : public Registrable<MulImplBackward_cpu, std::tuple<DataType, DataType, DataType>, void(const std::vector<std::size_t>&, const std::vector<std::size_t>&, const std::vector<std::size_t>&, const void*, const void*, void*)> {
-};
+    : public Registrable<MulImplBackward_cpu, std::tuple<DataType, DataType, DataType>, 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 void*, 
+                                                                                             const void*, 
+                                                                                             const void*, 
+                                                                                             void*, 
+                                                                                             void*)> {};
+
 
 class MulImpl_cpu : public OperatorImpl {
 public:
@@ -40,7 +54,9 @@ public:
     }
 
     Elts_t getNbRequiredProtected(const IOIndex_t inputIdx) const override final;
+
     void forward() override;
+    void backward() override;
 };
 
 namespace {