change Backend argument type for a std::string

include/aidge/backend/cuda/data/TensorImpl.hpp

 #ifndef AIDGE_BACKEND_CUDA_DATA_TENSORIMPL_H_
 #define AIDGE_BACKEND_CUDA_DATA_TENSORIMPL_H_
 
+#include <cstddef>  // std::size_t
+#include <memory>
+#include <string>
+
 #include "aidge/backend/TensorImpl.hpp"
 #include "aidge/data/Tensor.hpp"
 #include "aidge/utils/Registrar.hpp"
@@ -67,10 +71,10 @@ private:
     std::unique_ptr<T, decltype(&cudaDelete)> mDataOwner;
 
 public:
-    static constexpr const char *Backend = "cuda";
+    static const std::string Backend;
 
     TensorImpl_cuda(DeviceIdx_t device, std::vector<DimSize_t> dims) : TensorImpl(Backend, device, dims), mDataOwner(nullptr, cudaDelete) {}
-    
+
 
     bool operator==(const TensorImpl &otherImpl) const override final;
 
@@ -234,6 +238,9 @@ private:
     }
 };
 
+template <typename T>
+const std::string TensorImpl_cuda<T>::Backend = "cuda";
+
 namespace {
 static Registrar<Tensor> registrarTensorImpl_cuda_Float64(
         {"cuda", DataType::Float64}, Aidge::TensorImpl_cuda<double>::create);