Pseudo-Label Training and Model Inertia in Neural Machine Translation

However, these models have been observed to be brittle: NMT model predictions are sensitive to small input changes and can show significant variation across re-training or incremental model updates. This work studies a frequently used method in NMT, pseudo-label training (PLT), which is common to the related techniques of forward-translation (or self-training) and sequence-level knowledge distillation. While the effect of PLT on quality is well-documented, we highlight a lesserknown effect: PLT can enhance a model's stability to model updates and input perturbations, a set of properties we call model inertia. We study inertia effects under different training settings and we identify distribution simplification as a mechanism behind the observed results. Self-training (Fralick, 1967; Amini et al., 2022) is a popular semi-supervised technique used to boost the performance of neural machine translation (NMT) models. In self-training for NMT, also known as forward-translation, an initial model is used to translate monolingual data; this data is then concatenated with the original training data in a subsequent training step (Zhang & Zong, 2016; Marie et al., 2020; Edunov et al., 2020; Wang et al., 2021). Self-training is believed to be effective through inducing input smoothness and leading to better learning of decision boundaries from the addition of unlabeled data (Chapelle et al., 2006; He et al., 2020; Wei et al., 2021). It has also been observed to effectively diversify the training distribution (Wang et al., 2021; Nguyen et al., 2020). A closely related technique is that of knowledge distillation (Hinton et al., 2015; Gou et al., 2021), particularly sequence-level knowledge distillation (SKD), which uses hard targets in training and reduces to pseudo-labeled data augmentation (Kim & Rush, 2016). In NMT, knowledge distillation is effective through knowledge transfer from ensembles or larger-capacity models and as a data augmentation method (Freitag et al., 2017; Gordon & Duh, 2019; Tan et al., 2019; Currey et al., 2020).