  Open-domain dialog generation is a challenging problem; maximum likelihood
training can lead to repetitive outputs, models have difficulty tracking
long-term conversational goals, and training on standard movie or online
datasets may lead to the generation of inappropriate, biased, or offensive
text. Reinforcement Learning (RL) is a powerful framework that could
potentially address these issues, for example by allowing a dialog model to
optimize for reducing toxicity and repetitiveness. However, previous approaches
which apply RL to open-domain dialog generation do so at the word level, making
it difficult for the model to learn proper credit assignment for long-term
conversational rewards. In this paper, we propose a novel approach to
hierarchical reinforcement learning, VHRL, which uses policy gradients to tune
the utterance-level embedding of a variational sequence model. This
hierarchical approach provides greater flexibility for learning long-term,
conversational rewards. We use self-play and RL to optimize for a set of
human-centered conversation metrics, and show that our approach provides
significant improvements -- in terms of both human evaluation and automatic
metrics -- over state-of-the-art dialog models, including Transformers.
