[bugfix]修复Tester初始化bug

[ouyhlan]

Description：修复Tester初始化bug

Main reason: core/tester.py里Tester类的初始化方法有一个很明显的bug https://github.com/fastnlp/fastNLP/blob/3cb01d15d8bc7d10f292b12e8fa803087d37e887/fastNLP/core/tester.py#L151 这里应该改为

            if _model_contains_inner_module(self._model):

因为 https://github.com/fastnlp/fastNLP/blob/3cb01d15d8bc7d10f292b12e8fa803087d37e887/fastNLP/core/tester.py#L111 且它上下文里其他部分： https://github.com/fastnlp/fastNLP/blob/3cb01d15d8bc7d10f292b12e8fa803087d37e887/fastNLP/core/tester.py#L135-L156 都用的是self._model

Checklist 检查下面各项是否完成

Please feel free to remove inapplicable items for your PR.

• [x] The PR title starts with [$CATEGORY] (例如[bugfix]修复bug，[new]添加新功能，[test]修改测试，[rm]删除旧代码)
• [x] Changes are complete (i.e. I finished coding on this PR) 修改完成才提PR
• [x] All changes have test coverage 修改的部分顺利通过测试。对于fastnlp/fastnlp/的修改，测试代码必须提供在fastnlp/test/。
• [x] Code is well-documented 注释写好，API文档会从注释中抽取
• [x] To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change 修改导致例子或tutorial有变化，请找核心开发人员

Changes:

• 修复Tester类初始化bug

Mention:

@yhcc