Toward Adversarial Training on Contextualized Language Representation
Hongqiu Wu Yongxiang Liu Hanwen Shi Hai Zhao Min Zhang
Beyond the success story of adversarial training (AT) in the recent text
domain on top of pre-trained language models (PLMs), our empirical study
showcases the inconsistent gains from AT on some tasks, e.g. commonsense
reasoning, named entity recognition. This paper investigates AT from the
perspective of the contextualized language representation outputted by PLM
encoders. We find the current AT attacks lean to generate sub-optimal
adversarial examples that can fool the decoder part but have a minor effect on
the encoder. However, we find it necessary to effectively deviate the latter
one to allow AT to gain. Based on the observation, we propose simple yet
effective \textit{Contextualized representation-Adversarial Training} (CreAT),
in which the attack is explicitly optimized to deviate the contextualized
representation of the encoder. It allows a global optimization of adversarial
examples that can fool the entire model. We also find CreAT gives rise to a
better direction to optimize the adversarial examples, to let them less
sensitive to hyperparameters. Compared to AT, CreAT produces consistent
performance gains on a wider range of tasks and is proven to be more effective
for language pre-training where only the encoder part is kept for downstream
tasks. We achieve the new state-of-the-art performances on a series of
challenging benchmarks, e.g. AdvGLUE (59.1 $ \rightarrow $ 61.1), HellaSWAG
(93.0 $ \rightarrow $ 94.9), ANLI (68.1 $ \rightarrow $ 69.3).