Improve Sigmoid

Context

Improve Sigmoid implementation for CPU backend to avoid numerical instabilities

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

@@ -27,7 +27,11 @@ void SigmoidImpl_cpu_forward_kernel(std::size_t inputLenght,
 
 //#pragma omp parallel for if (inputLenght > 1024)
     for (std::size_t i = 0; i < inputLenght; ++i) {
-        output[i] = O(1) / (O(1) + std::exp(-input[i]));
+		if (input[i] > I(0)) {
+			output[i] = O(1) / (O(1) + std::exp(-input[i]));
+		} else {
+			output[i] = std::exp(input[i]) / (O(1) + std::exp(input[i]));
+		}
     }
 }