  Despite exciting progress in pre-training for visual-linguistic (VL)
representations, very few aspire to a small VL model. In this paper, we study
knowledge distillation (KD) to effectively compress a transformer-based large
VL model into a small VL model. The major challenge arises from the
inconsistent regional visual tokens extracted from different detectors of
Teacher and Student, resulting in the misalignment of hidden representations
and attention distributions. To address the problem, we retrain and adapt the
Teacher by using the same region proposals from Student's detector while the
features are from Teacher's own object detector. With aligned network inputs,
the adapted Teacher is capable of transferring the knowledge through the
intermediate representations. Specifically, we use the mean square error loss
to mimic the attention distribution inside the transformer block and present a
token-wise noise contrastive loss to align the hidden state by contrasting with
negative representations stored in a sample queue. To this end, we show that
our proposed distillation significantly improves the performance of small VL
models on image captioning and visual question answering tasks. It reaches
120.8 in CIDEr score on COCO captioning, an improvement of 5.1 over its
non-distilled counterpart; and an accuracy of 69.8 on VQA 2.0, a 0.8 gain from
the baseline. Our extensive experiments and ablations confirm the effectiveness
of VL distillation in both pre-training and fine-tuning stages.
