`get_final_text` bug when dealing with chinese sentence
Hi,

I set `max_answer_length` to `30`, but I still got really long answers, so I print the `tok_text`, `orig_text` and `final_text` in function `write_predictions`. 

```
tok_text: 权 健 公 司 可 能 涉 及 的 刑 事 罪 名 是 否 仅 仅 是 [UNK] 虚 假 广 告 罪
orig_text: 根据相关法律，权健公司可能涉及的刑事罪名是否仅仅是“虚假广告罪”“组织、领导传销活动罪”两个罪名，应该说仍有不少需要进一步深入调查的空间
final_text: 根据相关法律，权健公司可能涉及的刑事罪名是否仅仅是“虚假广告罪”“组织、领导传销活动罪”两个罪名，应该说仍有不少需要进一步深入调查的空间
```

From `tok_text` we can see that the answer should be `权健公司可能涉及的刑事罪名是否仅仅是“虚假广告罪`, however, `final_text` we got turned out to be much longer.

Perhaps it's the problem of tokenizer...After stripping the lengths of two sequences changed, so `orig_text` is returned...<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
