Add backward functions for ReLU, Sigmoid and Tanh

include/aidge/backend/cpu/operator/ReLUImpl_backward_kernels.hpp

@@ -22,8 +22,8 @@ template <class I, class O, class GI, class GO>
 void ReLUImpl_cpu_backward_kernel(const std::size_t inputLenght,
                                   const void* input_, const void* output_, const void* grad_output_,
 								  void* grad_input_) {
-	const I* input = static_cast<const I*>(input_);
-    const O* output = static_cast<const O*>(output_);
+    const I* input = static_cast<const I*>(input_);
+    #const O* output = static_cast<const O*>(output_);
     const GO* grad_output = static_cast<const GO*>(grad_output_);
     GI* grad_input = static_cast<GI*>(grad_input_);
     for (std::size_t i = 0; i < inputLenght; ++i) {

include/aidge/backend/cpu/operator/SigmoidImpl_backward_kernels.hpp

@@ -22,7 +22,7 @@ template <class I, class O, class GI, class GO>
 void SigmoidImpl_cpu_backward_kernel(const std::size_t inputLenght,
                                      const void* input_, const void* output_, const void* grad_output_,
 								     void* grad_input_) {
-	const I* input = static_cast<const I*>(input_);
+    #const I* input = static_cast<const I*>(input_);
     const O* output = static_cast<const O*>(output_);
     const GO* grad_output = static_cast<const GO*>(grad_output_);
     GI* grad_input = static_cast<GI*>(grad_input_);

include/aidge/backend/cpu/operator/TanhImpl_backward_kernels.hpp

@@ -22,7 +22,7 @@ template <class I, class O, class GI, class GO>
 void TanhImpl_cpu_backward_kernel(const std::size_t inputLenght,
                                   const void* input_, const void* output_, const void* grad_output_,
 			  			          void* grad_input_) {
-	const I* input = static_cast<const I*>(input_);
+    #const I* input = static_cast<const I*>(input_);
     const O* output = static_cast<const O*>(output_);
     const GO* grad_output = static_cast<const GO*>(grad_output_);
     GI* grad_input = static_cast<GI*>(grad_input_);