  Retrieval-enhanced text generation has shown remarkable progress on
knowledge-intensive language tasks, such as open-domain question answering and
knowledge-enhanced dialogue generation, by leveraging passages retrieved from a
large passage corpus for delivering a proper answer given the input query.
However, the retrieved passages are not ideal for guiding answer generation
because of the discrepancy between retrieval and generation, i.e., the
candidate passages are all treated equally during the retrieval procedure
without considering their potential to generate a proper answer. This
discrepancy makes a passage retriever deliver a sub-optimal collection of
candidate passages to generate the answer. In this paper, we propose the
GeneRative Knowledge Improved Passage Ranking (GripRank) approach, addressing
the above challenge by distilling knowledge from a generative passage estimator
(GPE) to a passage ranker, where the GPE is a generative language model used to
measure how likely the candidate passages can generate the proper answer. We
realize the distillation procedure by teaching the passage ranker learning to
rank the passages ordered by the GPE. Furthermore, we improve the distillation
quality by devising a curriculum knowledge distillation mechanism, which allows
the knowledge provided by the GPE can be progressively distilled to the ranker
through an easy-to-hard curriculum, enabling the passage ranker to correctly
recognize the provenance of the answer from many plausible candidates. We
conduct extensive experiments on four datasets across three knowledge-intensive
language tasks. Experimental results show advantages over the state-of-the-art
methods for both passage ranking and answer generation on the KILT benchmark.
