fix layer/node topo sort problem (#2433)

* fix layer/node topo sort problem

* fix to only iterate over valid layer/node keys

keras/engine/topology.py

@@ -1672,7 +1672,8 @@ def build_map_of_graph(tensor, seen_nodes=set(), depth=0,
                 layers_by_depth[depth] = []
             layers_by_depth[depth].append(layer)
 
-        depth_keys = list(nodes_by_depth.keys())
+        # get sorted list of layer depths
+        depth_keys = list(layers_by_depth.keys())
         depth_keys.sort(reverse=True)
 
         # set self.layers and self.layers_by_depth
@@ -1686,6 +1687,10 @@ def build_map_of_graph(tensor, seen_nodes=set(), depth=0,
         self.layers = layers
         self.layers_by_depth = layers_by_depth
 
+        # get sorted list of node depths
+        depth_keys = list(nodes_by_depth.keys())
+        depth_keys.sort(reverse=True)
+
         # check that all tensors required are computable.
         # computable_tensors: all tensors in the graph
         # that can be computed from the inputs provided