[Upd] Shape so that 'forwardDims()' does not require a backend anymore

Update shape so that forwardDims does not require a backend. Update test to remove unecessary Node.

include/aidge/data/Tensor.hpp

@@ -393,6 +393,13 @@ public:
         return hasImpl() ? getImpl()->backend() : "";
     }
 
+
+    /**
+     * @brief Get the device index.
+     * @return DeviceIdx_t
+     */
+    DeviceIdx_t device() const noexcept { return mImpl ? mImpl->device().second : static_cast<DeviceIdx_t>(0); }
+
     /**
      * @brief Set the backend of the Tensor associated implementation. If there
      * was no previous implementation set, data will be allocated, but it will