text: Title: Extend bert/embedding.py with other pre-trained models 
  Body: We shall extend bert/embedding.py with other pre-trained models like XLNet, RoBERTa, R-XLM, ALBERT, etc. This is a good starting script where people try to leverage pre-trained model embeddings for their models for various use cases. @leezu this is somewhat aligned with the efforts for refactoring data preprocessing. I am also ok if we make copy-pastable embedding.py for other pre-trained models. 