A Flexible BERT Model Enabling Width- and Depth-Dynamic Inference

Fine-tuning and inference on Large Language Models like BERT have become increasingly expensive regarding memory cost and computation resources. The recently proposed computation-flexible BERT models facilitate their deployment in varied computational environments. Training such flexible BERT models involves jointly optimizing multiple BERT subnets, which will unavoidably interfere with one another. Besides, the performance of large subnets is limited by the performance gap between the smallest subnet and the supernet, despite efforts to enhance the smaller subnets. In this regard, we propose layer-wise Neural grafting to boost BERT subnets, especially the larger ones. The proposed method improves the average performance of the subnets on six GLUE tasks and boosts the supernets on all GLUE tasks and the SQuAD data set. Based on the boosted subnets, we further build an inference framework enabling practical width- and depth-dynamic inference regarding different inputs by combining width-dynamic gating modules and early exit off-ramps in the depth dimension. Experimental results show that the proposed framework achieves a better dynamic inference range than other methods in terms of trading off performance and computational complexity on four GLUE tasks and SQuAD. In particular, our best-tradeoff inference result outperforms other fixed-size models with similar amount of computations. Compared to BERT-Base, the proposed inference framework yields a 1.3-point improvement in the average GLUE score and a 2.2-point increase in the F1 score on SQuAD, while reducing computations by around 45%.