Allow number of nodes in CUDA graph to change (#7738)

Previously the code would have failed to cope in the case that the
number of nodes changes in an existing CUDA graph. This fixes the
issue by removing an unnecessary conditional.

ggml-cuda.cu

@@ -2702,10 +2702,8 @@ GGML_CALL static enum ggml_status ggml_backend_cuda_graph_compute(ggml_backend_t
 
         if (cuda_graph_update_required) {
             // Extract nodes from graph
-            if (cuda_ctx->cuda_graph->num_nodes == 0) {
-                // First call with null argument gets number of nodes in graph
-                CUDA_CHECK(cudaGraphGetNodes(cuda_ctx->cuda_graph->graph, nullptr, &cuda_ctx->cuda_graph->num_nodes));
-            }
+            // First call with null argument gets number of nodes in graph
+            CUDA_CHECK(cudaGraphGetNodes(cuda_ctx->cuda_graph->graph, nullptr, &cuda_ctx->cuda_graph->num_nodes));
             // Subsequent call with non-null argument gets nodes
             cuda_ctx->cuda_graph->nodes.resize(cuda_ctx->cuda_graph->num_nodes);
             cuda_ctx->cuda_graph->params.resize(cuda_ctx->cuda_graph->num_nodes);