fix bug in py3 infer (#2913)

tools/infer.py

@@ -109,12 +109,14 @@ def save_infer_model(FLAGS, exe, feed_vars, test_fetches, infer_prog):
     cfg_name = os.path.basename(FLAGS.config).split('.')[0]
     save_dir = os.path.join(FLAGS.output_dir, cfg_name)
     feeded_var_names = [var.name for var in feed_vars.values()]
-    target_vars = test_fetches.values()
-    feeded_var_names = prune_feed_vars(feeded_var_names, target_vars, infer_prog)
+    target_vars = list(test_fetches.values())
+    feeded_var_names = prune_feed_vars(feeded_var_names, target_vars,
+                                       infer_prog)
     logger.info("Save inference model to {}, input: {}, output: "
                 "{}...".format(save_dir, feeded_var_names,
                                [var.name for var in target_vars]))
-    fluid.io.save_inference_model(save_dir, 
+    fluid.io.save_inference_model(
+        save_dir,
         feeded_var_names=feeded_var_names,
         target_vars=target_vars,
         executor=exe,