Description：简要描述这次PR的内容
Bugfix embed_loader.py
Main reason: 做出这次修改的原因
Some lines in the glove file do not contain an embedded word. There're only embedding vectors in these lines. For these lines, len(line) equals to 'emb_dim', rather than 'emb_dim + 1'. Just setting 'if len(line) > 2' won't work for all the lines.

Checklist 检查下面各项是否完成

Please feel free to remove inapplicable items for your PR.

• 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)
• Changes are complete (i.e. I finished coding on this PR) 修改完成才提PR
• 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/。
• Code is well-documented 注释写好，API文档会从注释中抽取
• 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: 逐项描述修改的内容

• modify 'if len(line) > 2' to 'if len(line) == emb_dim + 1'
• add parameter 'emb_dim' in function '_load_pretrain()' and '_load_glove()'

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