HEARTS: Multi-task Fusion of Dense Retrieval and Non-autoregressive Generation for Sponsored Search

Matching user search queries with relevant keywords bid by advertisers in real-time is a crucial problem in sponsored search. In the literature, two broad set of approaches have been explored to solve this problem: (i) Dense Retrieval (DR) - learning dense vector representations for queries and bid keywords in a shared space, and (ii) Natural Language Generation (NLG) - learning to directly generate bid keywords given queries. In this work, we first conduct an empirical study of these two approaches and show that they offer complementary benefits that are additive. In particular, a large fraction of the keywords retrieved from NLG haven't been retrieved by DR and vice-versa. We then show that it is possible to effectively combine the advantages of these two approaches in one model. Specifically, we propose HEARTS: a novel multi-task fusion framework where we jointly optimize a shared encoder to perform both DR and non-autoregressive NLG. Through extensive experiments on search queries from over 30+ countries spanning 20+ languages, we show that HEARTS retrieves 40.3% more high-quality bid keywords than the baseline approaches with the same GPU compute. We also demonstrate that inferring on a single HEARTS model is as good as inferring on two different DR and NLG baseline models, with 2x the compute. Further, we show that DR models trained with the HEARTS objective are significantly better than those trained with the standard contrastive loss functions. Finally, we show that our HEARTS objective can be adopted to short-text retrieval tasks other than sponsored search and achieve significant performance gains.