Relevance-guided Supervision for OpenQA with ColBERT Omar Khattab author Christopher Potts author Matei Zaharia author 2021 text journal article Transactions of the Association for Computational Linguistics continuing MIT Press Cambridge, MA periodical academic journal khattab-etal-2021-relevance 10.1162/tacl_a_00405 https://aclanthology.org/2021.tacl-1.55/ 2021 9 929 944