fix mask token

app.py

@@ -219,7 +219,7 @@ if __name__=='__main__':
     if st.session_state['page_status']!='model_selection':
         tokenizer,model,skeleton_model = load_model(st.session_state['model_name'])
         num_layers, num_heads = model.config.num_hidden_layers, model.config.num_attention_heads
-        mask_id = tokenizer('[MASK]').input_ids[1:-1][0]
+        mask_id = tokenizer(tokenizer.mask_token).input_ids[1:-1][0]
 
     if st.session_state['page_status']=='type_in':
         show_instruction('1. Type in the sentences and click "Tokenize"',fontsize=16)