feat: Backward of Stack Operator

Added the backward pass implementation for the Stack operator. The backward pass extract gradients from the stacked output grad.

Key changes:

• Implemented StackOpImpl::backward() method to handle gradient propagation
• Added BackwardStep counter attribute to track backward pass position
• Extended unit tests to verify gradient computation

include/aidge/operator/Stack.hpp

@@ -89,10 +89,16 @@ public:
      * @brief Executes the forward pass for the Stack operation.
      */
     void forward() override;
+
+    /**
+     * @brief Executes the backward pass for the Stack operation.
+     */
+    void backward() override;
 };
 
 enum class StackAttr {
     ForwardStep,   // Tracks the current step in the forward pass.
+    BackwardStep,  // Tracks the current step in the forward pass.
     MaxElements    // Maximum number of elements that can be stacked.
 };
 }  // namespace Aidge
@@ -123,7 +129,7 @@ namespace Aidge {
 class StackOp : public OperatorTensor,
     public Registrable<StackOp, std::string, std::function<std::unique_ptr<OperatorImpl>(const StackOp&)>> {
 private:
-    using Attributes_ = StaticAttributes<StackAttr, std::uint32_t, std::uint32_t>;
+    using Attributes_ = StaticAttributes<StackAttr, std::uint32_t, std::uint32_t, std::uint32_t>;
     template <StackAttr e> using attr = typename Attributes_::template attr<e>;
     const std::shared_ptr<Attributes_> mAttributes;
 
@@ -181,6 +187,11 @@ public:
      */
     void forward() override;
 
+    /**
+     * @brief Executes the backward pass for the `Stack` operation.
+     */
+    void backward() override;
+
     /**
      * @brief Access the operator's attributes.
      * @return A shared pointer to the operator's attributes.
@@ -205,6 +216,15 @@ public:
         return mAttributes->template getAttr<StackAttr::ForwardStep>();
     }
 
+    /**
+     * @brief Get or set the backward step counter for the operator.
+     * @return A reference to the backward step counter.
+     */
+    inline std::uint32_t& backwardStep() const {
+        return mAttributes->template getAttr<StackAttr::BackwardStep>();
+    }
+
+
     /**
      * @brief Retrieve the names of the operator's input tensors.
      * @return A vector of strings representing input tensor names.
@@ -239,5 +259,4 @@ public:
 std::shared_ptr<Node> Stack(std::uint32_t maxElements = 0, const std::string& name = "");
 }  // namespace Aidge
 
-
 #endif /* AIDGE_CORE_OPERATOR_STACK_H_ */