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

src/operator/Stack.cpp

@@ -61,7 +61,18 @@ void StackOpImpl::forward() {
         op.forwardStep() * op.getInput(0)->size());
 }
 
-void StackOpImpl::backward() {}
+void StackOpImpl::backward() {
+    const StackOp &op = dynamic_cast<const StackOp &>(mOp);
+    AIDGE_ASSERT(op.backwardStep() > 0, "Stack operator has not been run forward");
+
+    auto inputGrad = op.getInput(0)->grad();
+    auto outputGrad = op.getOutput(0)->grad();
+
+    Log::notice("Size of stack in grad  : {}", inputGrad->size());
+    Log::notice("Size of stack out grad : {}", outputGrad->size());
+
+    *inputGrad = outputGrad->extract({op.backwardStep() -1 }).clone();
+}
 
 StackOp::StackOp(std::uint32_t maxElements)
     : OperatorTensor(s_type, {InputCategory::Data, InputCategory::OptionalData}, 1),
@@ -137,9 +148,13 @@ std::set<std::string> StackOp::getAvailableBackends() const {
 void StackOp::forward() {
     OperatorTensor::forward();
     ++forwardStep();
+    backwardStep() = forwardStep();
 }
 
-void StackOp::backward() {}
+void StackOp::backward() {
+    OperatorTensor::backward();
+    --backwardStep();
+}
 
 std::shared_ptr<Node> Stack(std::uint32_t maxElements,
                             const std::string &name) {