  Multi-party dialogues are more difficult for models to understand than
one-to-one two-party dialogues, since they involve multiple interlocutors,
resulting in interweaving reply-to relations and information flows. To step
over these obstacles, an effective way is to pre-train a model that understands
the discourse structure of multi-party dialogues, namely, to whom each
utterance is replying. However, due to the lack of explicitly annotated
discourse labels in multi-party dialogue corpora, previous works fail to scale
up the pre-training process by putting aside the unlabeled multi-party
conversational data for nothing. To fully utilize the unlabeled data, we
propose to treat the discourse structures as latent variables, then jointly
infer them and pre-train the discourse-aware model by unsupervised latent
variable inference methods. Experiments on multiple downstream tasks show that
our pre-trained model outperforms strong baselines by large margins and
achieves state-of-the-art (SOTA) results, justifying the effectiveness of our
method. The official implementation of this paper is available at
https://github.com/EricLee8/MPD_EMVI.
