Refactor OperatorImpl for backend/export

Refactor OperatorImpl for backend/export.

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

@@ -18,11 +18,11 @@ namespace Aidge {
 
 template <class I, class O>
 void MatMulImpl_cpu_forward_kernel(const std::size_t n, const std::size_t k, const std::size_t m,
-                                    const void* input1_, const void* input2_, void* __restrict__ output_) {
+                                    const void* input1_, const void* input2_, void* __restrict output_) {
     // FIXME: missing MatMul parameters as arguments
     const I* input1 = static_cast<const I*>(input1_);
     const I* input2 = static_cast<const I*>(input2_);
-    O* __restrict__ output = static_cast<O* __restrict__>(output_);
+    O* __restrict output = static_cast<O* __restrict>(output_);
 
     std::memset(output, O(0), n * m * sizeof(O));