Add: backward pass for Pop operator

Added the backward pass implementation for the Pop operator. The backward pass stacks gradients in the correct positions of the input tensor based on the pop sequence.

Key changes:

• Implemented Pop_OpImpl::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/Pop.hpp

@@ -86,6 +86,11 @@ public:
      * @brief Executes the forward pass for the `Pop` operation.
      */
     void forward() override;
+
+    /**
+     * @brief Executes the backward pass for the `Pop` operation.
+     */
+    void backward() override;
 };
 
 /**
@@ -93,7 +98,8 @@ public:
  * @brief Attributes specific to the `Pop` operator.
  */
 enum class PopAttr {
-    ForwardStep     // Tracks the current step in the forward pass
+    ForwardStep,    // Tracks the current step in the forward pass
+    BackwardStep    // Tracks the current step in the backward pass
 };
 
 /**
@@ -115,7 +121,7 @@ public:
     static const std::string Type;
 
 private:
-    using Attributes_ = StaticAttributes<PopAttr, std::uint32_t>;
+    using Attributes_ = StaticAttributes<PopAttr, std::uint32_t, std::uint32_t>;
     template <PopAttr e> using attr = typename Attributes_::template attr<e>;
     const std::shared_ptr<Attributes_> mAttributes;
 
@@ -171,6 +177,11 @@ public:
      */
     void forward() override;
 
+    /**
+     * @brief Executes the backward pass for the `Pop` operation.
+     */
+    void backward() override;
+
     /**
      * @brief Access the operator's attributes.
      * @return A shared pointer to the operator's attributes.
@@ -183,6 +194,8 @@ public:
      */
     inline std::uint32_t& forwardStep() const { return mAttributes->template getAttr<PopAttr::ForwardStep>(); }
 
+    inline std::uint32_t& backwardStep() const { return mAttributes->template getAttr<PopAttr::BackwardStep>(); }
+
     /**
      * @brief Retrieve the names of the operator's input tensors.
      * @return A vector of strings representing input tensor names.
@@ -214,7 +227,7 @@ namespace {
  */
 template <>
 const char *const EnumStrings<Aidge::PopAttr>::data[] = {
-    "forward_step"
+    "forward_step", "backward_step"
 };
 }