Add "GLEN: Generative Retrieval via Lexical Index Learning" and "Enhancing Generative Retrieval with Reinforcement Learning from Relevance Feedback"

[skleee]

• add GLEN: Generative Retrieval via Lexical Index Learning (EMNLP 2023)
• add Enhancing Generative Retrieval with Reinforcement Learning from Relevance Feedback (EMNLP 2023)