add paper citation

ppcls/arch/backbone/model_zoo/van.py

-# copyright (c) 2021 PaddlePaddle Authors. All Rights Reserve.
+# copyright (c) 2022 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.
@@ -185,6 +185,11 @@ class OverlapPatchEmbed(nn.Layer):
 
 
 class VAN(nn.Layer):
+    r""" VAN
+    A PaddlePaddle impl of : `Visual Attention Network`  -
+      https://arxiv.org/pdf/2202.09741.pdf
+    """
+
     def __init__(self,
                  img_size=224,
                  in_chans=3,