Paper:
Language model pre-training, such as BERT, has significantly improved the performances of many natural language processing tasks. However, the pre-trained language models are usually computationally expensive and memory intensive, so it is difficult to effectively execute them on resource-restricted devices. To accelerate inference and reduce model size while maintaining accuracy, we firstly propose a novel Transformer distillation method that is specially designed for knowledge distillation (KD) of the Transformer-based models. By leveraging this new KD method, the plenty of knowledge encoded in a large "teacher" BERT can be well transferred to a small "student" TinyBERT. Moreover, we introduce a new two-stage learning framework for TinyBERT, which performs Transformer distillation at both the pre-training and task-specific learning stages. This framework ensures that TinyBERT can capture the general-domain as well as the task-specific knowledge in BERT. TinyBERT 1 is empirically effective and achieves more than 96% the performance of teacher BERT BASE on GLUE benchmark, while being 7.5x smaller and 9.4x faster on inference. TinyBERT is also significantly better than state-of-the-art baselines on BERT distillation, with only ∼28% parameters and ∼31% inference time of them.

Review:
This paper proposes a new knowledge distillation method for BERT models. A number of modifications to the vanilla knowledge distillation method of Hinton et al (2015) are proposed. First, authors suggest adding L2 loss functions between alignment matrices, embedding layer values and prediction layer values. Second, authors propose run knowledge-distillation twice, once with the original pre-trained BERT model as teacher, and then again with task specific fine-tuned BERT as a new teacher. Third, authors emphasize the use of data augmentation for successful knowledge distillation. In Table 2, authors claim a significant lift across GLUE benchmarks with respect to other baseline methods with comparable model size.
While the main contribution of this paper is the proposal of empirically useful techniques than theoretical development, the empirical results reported in this paper are somewhat puzzling. 
First of all, GLUE benchmark scores reported in Table 2 don't seem to be consistent with Table 1 of Sun et al (2019) for BERT-PKD ( https://arxiv.org/pdf/1908.09355.pdf ) or DistilBERT ( https://medium.com/huggingface/distilbert-8cf3380435b5 ). Indeed, BERT-PKD in Sun et al seems to significantly outperform TinyBERT on QNLI (89.0 vs 87.7) and RTE (65.5 vs 62.9), and the gap between BERT-PKD and TinyBERT on other tasks are much smaller if we take numbers reported in the original paper.
In Table 6, ablation studies with different distillation objectives are reported. Quite surprisingly, without Transformer-layer distillation (No Trm) the performance drops quite significantly. This is unexpected, because baselines such as Sun et al and DistilBERT do not use the Transformer-layer distillation but much more competitive to full TinyBERT than TinyBERT without Transformer-layer distillation. Would there be a reason why TinyBERT is so critically dependent on Transformer-layer distillation? Similarly, the removal of data augmentation (Table 5, No DA) is so detrimental to the performance of the model that it makes me to suspect whether the most of gain is from successful data augmentation. Indeed, 'No DA' row of Table 5 is very close to the performance BERT-PKD in Table 4, although the number of layers is different (4 vs 6). 
In order for the research community to understand the contribution of proposed techniques more thoroughly, I suggest authors to conduct ablation studies with the simplest baseline. That is, rather than starting with the full TinyBERT model, start with a simple but competitive baseline like BERT-PKD, and only add one technique (DA, GD, Transformer-layer distillation) at a time so that readers shall understand what technique is the most important to be added to the baseline, and also whether some of the proposed techniques should always be used in combination.
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After Author Rebuttal: authors have addressed all of my concerns quite clearly. Additional experiments which targeted a specific design choice at a time made me much more convinced that the techniques proposed in this paper are useful not only for this particular context but also more broadly applicable.