pytorch dpr transformers
This model is converted from the original BPR [repo](https://github.com/studio-ousia/bpr) and fitted into Pyserini:

> Ikuya Yamada, Akari Asai, and Hannaneh Hajishirzi. 2021. Efficient passage retrieval with hashing for open-domain question answering. arXiv:2106.00882.