  Pre-trained Language Models (PLMs) have achieved great success in various
Natural Language Processing (NLP) tasks under the pre-training and fine-tuning
paradigm. With large quantities of parameters, PLMs are computation-intensive
and resource-hungry. Hence, model pruning has been introduced to compress
large-scale PLMs. However, most prior approaches only consider task-specific
knowledge towards downstream tasks, but ignore the essential task-agnostic
knowledge during pruning, which may cause catastrophic forgetting problem and
lead to poor generalization ability. To maintain both task-agnostic and
task-specific knowledge in our pruned model, we propose ContrAstive Pruning
(CAP) under the paradigm of pre-training and fine-tuning. It is designed as a
general framework, compatible with both structured and unstructured pruning.
Unified in contrastive learning, CAP enables the pruned model to learn from the
pre-trained model for task-agnostic knowledge, and fine-tuned model for
task-specific knowledge. Besides, to better retain the performance of the
pruned model, the snapshots (i.e., the intermediate models at each pruning
iteration) also serve as effective supervisions for pruning. Our extensive
experiments show that adopting CAP consistently yields significant
improvements, especially in extremely high sparsity scenarios. With only 3%
model parameters reserved (i.e., 97% sparsity), CAP successfully achieves 99.2%
and 96.3% of the original BERT performance in QQP and MNLI tasks. In addition,
our probing experiments demonstrate that the model pruned by CAP tends to
achieve better generalization ability.
