Fix StackOp forward()

Call OperatorTensor::forward() instead of Operator::forward(), ensuring dims are
forwarded.

src/operator/Stack.cpp

@@ -132,7 +132,7 @@ std::set<std::string> StackOp::getAvailableBackends() const {
 }
 
 void StackOp::forward() {
-    Operator::forward();
+    OperatorTensor::forward();
     ++forwardStep();
 }