Update pan.py

Add source description for PANet

paddlers/models/ppgan/models/generators/pan.py

@@ -1,5 +1,4 @@
 #   Copyright (c) 2021 PaddlePaddle Authors. All Rights Reserve.
-#
 # 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
@@ -12,6 +11,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# code was based on https://github.com/zhaohengyuan1/PAN
+
 import functools
 import numpy as np