Add backward functions for ReLU, Sigmoid and Tanh

Context

Add missing backward functions for ReLU, Sigmoid and Tanh

TODO

This is a first try. Code needs to be validated and tested.

include/aidge/backend/cuda/operator/ReLUImpl.hpp

@@ -36,6 +36,7 @@ private:
         cudnnActivationMode_t mReLUDesc = nullptr;
     #endif
     std::shared_ptr<Tensor> mInputFallback;
+    std::shared_ptr<Tensor> mOutputGradFallback;
 
 public:
     ReLUImpl_cuda(const ReLU_Op &op) : OperatorImpl(op, "cuda") {}
@@ -46,10 +47,12 @@ public:
 
 public:
     void forward();
+    void backward();
     ~ReLUImpl_cuda();
 
 private:
     template <class T> void forward_(const Tensor& input);
+    template <class T> void backward_(const Tensor& output_grad);
 };
 
 namespace {