1) w/o low-level ED: without lowlevel encoder and low-level decoder; 2) w/o high-level ED: without high-level encoder and highlevel decoder; 3) w/o low&high-level ED: without low-level encoder/decoder and high-level encoder/decoder, which is the same as the Seq2Seq model with attention mechanisms.
vPN incorporates multipass encoding and decoding with multi-level vocabularies into response generation.