Make ReLUImpl_cpu members protected instead of private (need to extend this...

Make ReLUImpl_cpu members protected instead of private (need to extend this class in the module template example)

include/aidge/operator/ReLUImpl.hpp

@@ -31,7 +31,7 @@ class ReLUImplBackward_cpu
 };
 
 class ReLUImpl_cpu : public OperatorImpl {
-   private:
+   protected:
     const ReLU_Op& mOp;
     std::array<NbElts_t, 1> mNbConsumedData;
     std::array<NbElts_t, 1> mNbProducedData;