Implement a backward for Heaviside

Context

Since the Heaviside function is non-differentiable, the classical approach is to use a surrogate gradient for the backward pass. The most common gradient function used is the shifted arctan function :

For the forward pass, th :

S[t] = Heaviside(U[t] - U_{\rm thr}) \approx \frac{1}{π}\text{arctan}(\pi U) \\

Thus for the backward : \frac{\partial \tilde{S}}{\partial U} = \frac{1}{(1+(U \pi)^2)}

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

@@ -23,7 +23,7 @@ namespace Aidge {
 using HeavisideImplCpu =
     OperatorImpl_cpu<Heaviside_Op,
                      void(std::size_t, const void *, void *, const float),
-                     void(const float, std::size_t, const void *, void *)>;
+                     void(std::size_t, const void *, const void *, void *)>;
 
 // Implementation entry point registration for operator Heaviside
 REGISTRAR(Heaviside_Op, "cpu", HeavisideImplCpu::create);