Add a new API init_fan_in() to RRGraphBuilder

vpr/src/device/rr_graph_builder.h

@@ -122,6 +122,14 @@ class RRGraphBuilder {
         return node_storage_.count_rr_switches(num_arch_switches, arch_switch_inf, arch_switch_fanins);
     }
 
+    /** @brief Init per node fan-in data.  Should only be called after all edges have
+     * been allocated.
+     * @note
+     * This is an expensive, O(N), operation so it should be called once you
+     * have a complete rr-graph and not called often. */
+    inline void init_fan_in() {
+        node_storage_.init_fan_in();
+    }
     /* -- Internal data storage -- */
   private:
     /* TODO: When the refactoring effort finishes, 

vpr/src/route/rr_graph.cpp

@@ -1263,7 +1263,7 @@ static std::function<void(t_chan_width*)> alloc_and_load_rr_graph(RRGraphBuilder
         };
     }
 
-    L_rr_node.init_fan_in();
+    rr_graph_builder.init_fan_in();
 
     return update_chan_width;
 }

vpr/src/route/rr_graph_uxsdcxx_serializer.h

@@ -1547,7 +1547,7 @@ class RrGraphSerializer final : public uxsd::RrGraphBase<RrGraphContextTypes> {
     void finish_load() final {
         process_rr_node_indices();
 
-        rr_nodes_->init_fan_in();
+        rr_graph_builder_->init_fan_in();
 
         alloc_and_load_rr_indexed_data(
             segment_inf_,