Fix multiclass_nms_op unit test fail in python3.6

test=develop

python/paddle/fluid/tests/unittests/test_multiclass_nms_op.py

@@ -145,10 +145,16 @@ def batched_multiclass_nms(boxes, scores, background, score_threshold,
         lod.append(nmsed_num)
         if nmsed_num == 0: continue
 
+        tmp_det_out = []
         for c, indices in nmsed_outs.items():
             for idx in indices:
                 xmin, ymin, xmax, ymax = boxes[n][idx][:]
                 det_outs.append([c, scores[n][c][idx], xmin, ymin, xmax, ymax])
+                tmp_det_out.append(
+                    [c, scores[n][c][idx], xmin, ymin, xmax, ymax])
+        sorted_det_out = sorted(
+            tmp_det_out, key=lambda tup: tup[0], reverse=False)
+        det_outs.extend(sorted_det_out)
 
     return det_outs, lod
 
@@ -210,7 +216,7 @@ class TestMulticlassNMSOp(OpTest):
 class TestMulticlassNMSOpNoOutput(TestMulticlassNMSOp):
     def set_argument(self):
         # Here set 2.0 to test the case there is no outputs.
-        # In practical use, 0.0 < score_threshold < 1.0 
+        # In practical use, 0.0 < score_threshold < 1.0
         self.score_threshold = 2.0