add weights info in nextvlad

fluid/PaddleCV/video/models/nextvlad/nextvlad.py

@@ -147,6 +147,10 @@ class NEXTVLAD(ModelBase):
             self.label_input
         ]
     
+    def weights_info(self):
+        return ('nextvlad_youtube8m', 
+                'https://paddlemodels.bj.bcebos.com/video_classification/nextvlad_youtube8m.tar.gz')
+
 
 def get_learning_rate_decay_list(base_learning_rate, decay, max_iter,
                                  decay_examples, total_batch_size):