update 1.8 pre-trained language models and language models.

[Joejiong]

Update 1.8 pre-trained language models and language models.

1. 升级language_model模型内接口到paddle 1.8版本

2. 升级pretrain下bert和xlnet两个模块型内接口到paddle1.8版本
3. 
升级训练模型 -- language: 1个模型: ptb_lm -- pretrain_language_model: 2个模型: bert, xlnet

接口涉及：


1. save/load
2. set_gradient_clip
3. one_hot

4. embedding
5. 部分io接口

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:x: Joejiong

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