[cherry-pick] fix bbox_num 0, test=dygraph (#2201)

* fix bbox_num 0 after nms, test=dygraph

* add copyright, test=dygraph

dygraph/ppdet/modeling/post_process.py

+# Copyright (c) 2020 PaddlePaddle Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
 import numpy as np
 import paddle
 import paddle.nn as nn
@@ -40,6 +54,7 @@ class BBoxPostProcess(object):
             bbox_pred = paddle.to_tensor(
                 np.array(
                     [[-1, 0.0, 0.0, 0.0, 0.0, 0.0]], dtype='float32'))
+            bbox_num = paddle.to_tensor(np.array([1], dtype='int32'))
         return bbox_pred, bbox_num
 
     def get_pred(self, bboxes, bbox_num, im_shape, scale_factor):