[TensorImpl] add getRaw method.

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

@@ -47,6 +47,10 @@ class TensorImpl_cpu : public TensorImpl {
         return mData.data();
     };
 
+   void* getRaw(std::size_t idx){
+       return rawPtr() + (idx * sizeof(T));
+   };
+
     virtual ~TensorImpl_cpu() = default;
 
     void setRawPtr(void *ptr) override final {