Joe model 1.8 update

[Joejiong]

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：
 a) save/load 
b) DataLoader
 c) set_gradient_clip
 d) embedding e) one_hot
 f) 部分io接口；

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