1.CDSSM -- Based on https://www.microsoft.com/en-us/research/publication/a-convolutional-latent-semantic-model-for-web-search/
2.Seq2Seq -- Encoder: xletter, Decoder: term
3.QDocTreeRetrieve -- Based on the idea of Learning Tree-based Deep Model for Recommender Systems https://arxiv.org/pdf/1801.02294.pdf