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

@@ -68,9 +68,6 @@ void StackOpImpl::backward() {
     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();
 }