don't write down flat rr graph

Author: amin1377

vpr/src/route/rr_graph.cpp

@@ -794,7 +794,7 @@ void create_rr_graph(const t_graph_type graph_type,
     // When this function is called in any stage other than routing, the is_flat flag passed to this function is false, regardless of the flag passed
     // through command line. So, the graph corresponding to global resources will be created and written down to file if needed. During routing, if flat-routing
     // is enabled, intra-cluster resources will be added to the graph, but this new bigger graph will not be written down.
-    if (!det_routing_arch->write_rr_graph_filename.empty()) {
+    if (!det_routing_arch->write_rr_graph_filename.empty() && !is_flat) {
         write_rr_graph(&mutable_device_ctx.rr_graph_builder,
                        &mutable_device_ctx.rr_graph,
                        device_ctx.physical_tile_types,