UniKD: Universal Knowledge Distillation for Mimicking Homogeneous or Heterogeneous Object Detectors

Knowledge distillation (KD) has become a standard method to boost the performance of lightweight object detectors. Most previous works are feature-based, where students mimic the features of homogeneous teacher detectors. However, distilling the knowledge from the heterogeneous teacher fails in this manner due to the serious semantic gap, which greatly limits the flexibility of KD in practical applications. Bridging this semantic gap now requires case-by-case algorithm design which is time-consuming and heavily relies on experienced adjustment. To alleviate this problem, we propose Universal Knowledge Distillation (UniKD), introducing additional decoder heads with deformable cross-attention called Adaptive Knowledge Extractor (AKE). In UniKD, AKEs are first pretrained on the teacher's output to infuse the teacher's content and positional knowledge into a fixed-number set of knowledge embeddings. The fixed AKEs are then attached to the student's backbone to encourage the student to absorb the teacher's knowledge in these knowledge embeddings. In this query-based distillation paradigm, detection-relevant information can be dynamically aggregated into a knowledge embedding set and transferred between different detectors. When the teacher model is too large for online inference, its output can be stored on disk in advance to save the computation overhead, which is more storage efficient than feature-based methods. Extensive experiments demonstrate that our UniKD can plug and play in any homogeneous or heterogeneous teacher-student pairs and significantly outperforms conventional feature-based KD.