Improving Language Understanding by Generative Pre-Training
https://s3-us-west-2.amazonaws.com/openai-assets/research-covers/language-unsupervised/language_understanding_paper.pdf
OpenAI GPT-1の論文 2018
generativeの語の通り、事前学習でmaskの単語の推測を繰り返す（＝生成している）っぽい