  Multi-task learning (MTL) is to learn one single model that performs multiple
tasks for achieving good performance on all tasks and lower cost on
computation. Learning such a model requires to jointly optimize losses of a set
of tasks with different difficulty levels, magnitudes, and characteristics
(e.g. cross-entropy, Euclidean loss), leading to the imbalance problem in
multi-task learning. To address the imbalance problem, we propose a knowledge
distillation based method in this work. We first learn a task-specific model
for each task. We then learn the multi-task model for minimizing task-specific
loss and for producing the same feature with task-specific models. As the
task-specific network encodes different features, we introduce small
task-specific adaptors to project multi-task features to the task-specific
features. In this way, the adaptors align the task-specific feature and the
multi-task feature, which enables a balanced parameter sharing across tasks.
Extensive experimental results demonstrate that our method can optimize a
multi-task learning model in a more balanced way and achieve better overall
performance.
