Add member function 'Tensor::backend' to check the backend in Tensor

include/aidge/data/Tensor.hpp

@@ -348,6 +348,10 @@ public:
         return newTensor;
     }
 
+    const std::string backend() const {
+        return hasImpl() ? getImpl()->backend() : "";
+    }
+
     /**
      * @brief Set the backend of the Tensor associated implementation. If there
      * was no previous implementation set, data will be allocated, but it will
@@ -410,8 +414,8 @@ public:
      * @brief Get the Impl object
      * @return constexpr const std::shared_ptr<TensorImpl>&
      */
-    constexpr const std::shared_ptr<TensorImpl> &getImpl() const { return mImpl; }
-    constexpr std::size_t getImplOffset() const { return mImplOffset; }
+    constexpr const std::shared_ptr<TensorImpl>& getImpl() const noexcept { return mImpl; }
+    constexpr std::size_t getImplOffset() const noexcept { return mImplOffset; }
 
     /**
      * @brief Set the Impl object