Return tuple in retained_outputs

chainer/function_node.py

@@ -551,7 +551,7 @@ def get_retained_outputs(self):
         if outputs_modified:
             self.outputs = tuple(new_outputs)
 
-        return ret
+        return tuple(ret)
 
     def unchain(self):
         """Purges in/out nodes and this function node itself from the graph."""