knowledge_mining of nptag has a "list index out of range" error

[loahi]

• 版本、环境信息
  1）PaddleNLP和PaddlePaddle版本：
  PaddlePaddle 2.2.2, compiled with
  with_avx: ON
  with_gpu: OFF
  with_mkl: ON
  with_mkldnn: ON
  with_python: ON
  PaddleNLP 2.2.6

2）系统环境：
Linux jupyter-306844-3858354 4.4.0-166-generic #195-Ubuntu SMP Tue Oct 1 09:35:25 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Python 3.7.4

• 复现信息：如为报错，请给出复现环境、复现步骤
• 执行内容

from paddlenlp import Taskflow
nptag = Taskflow("knowledge_mining", model="nptag", linking=True)
nptag('支会')

• 错误信息

/opt/conda/envs/python35-paddle120-env/lib/python3.7/site-packages/paddlenlp/taskflow/taskflow.py in __call__(self, *inputs)
    289         The main work function in the taskflow.
    290         """
--> 291         results = self.task_instance(inputs)
    292         return results
    293 
/opt/conda/envs/python35-paddle120-env/lib/python3.7/site-packages/paddlenlp/taskflow/task.py in __call__(self, *args)
    270         inputs = self._preprocess(*args)
    271         outputs = self._run_model(inputs)
--> 272         results = self._postprocess(outputs)
    273         return results
/opt/conda/envs/python35-paddle120-env/lib/python3.7/site-packages/paddlenlp/taskflow/knowledge_mining.py in _postprocess(self, inputs)
    765                     else:
    766                         labels_can = self._tree.search_similar_word(cls_label)
--> 767                         result['label'] = labels_can[0][0]
    768 
    769             if self._linking:
IndexError: list index out of range

• 报错原因
  第766行 labels_can 返回的是一个空数组 []
  类似会返回空数组的词语还有“村合”，“世协”

[linjieccc]

感谢反馈！已PR修复：#2023