Fix update gradient tensor dimesions

include/aidge/data/Tensor.hpp

@@ -649,7 +649,7 @@ public:
     }
 
     void setGrad(std::shared_ptr<Tensor> newGrad) {
-        AIDGE_ASSERT(newGrad->dims() == mDims, "wrong dimensions");
+        AIDGE_ASSERT(!newGrad || newGrad->dims() == mDims, "wrong dimensions");
         mGrad = newGrad;
     }