  Multilingual sequence labeling is a task of predicting label sequences using
a single unified model for multiple languages. Compared with relying on
multiple monolingual models, using a multilingual model has the benefit of a
smaller model size, easier in online serving, and generalizability to
low-resource languages. However, current multilingual models still underperform
individual monolingual models significantly due to model capacity limitations.
In this paper, we propose to reduce the gap between monolingual models and the
unified multilingual model by distilling the structural knowledge of several
monolingual models (teachers) to the unified multilingual model (student). We
propose two novel KD methods based on structure-level information: (1)
approximately minimizes the distance between the student's and the teachers'
structure level probability distributions, (2) aggregates the structure-level
knowledge to local distributions and minimizes the distance between two local
probability distributions. Our experiments on 4 multilingual tasks with 25
datasets show that our approaches outperform several strong baselines and have
stronger zero-shot generalizability than both the baseline model and teacher
models.
