fix (#2613)

Co-authored-by: Chen-Chen Yeh <ge96noj@mytum.de>

mlx/backend/cuda/device.cpp

@@ -299,6 +299,7 @@ void CommandEncoder::add_graph_node(cudaGraph_t child) {
     graph_exec.instantiate(child);
     device_.make_current();
     CHECK_CUDA_ERROR(cudaGraphLaunch(graph_exec, stream()));
+    return;
   }
   cudaGraphNode_t node;
   CHECK_CUDA_ERROR(cudaGraphAddChildGraphNode(&node, graph_, NULL, 0, child));