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Title: ALBERT: A Lite BERT for Self-supervised Learning of Language Representations

Paper content:
Increasing model size when pretraining natural language representations often results in improved performance on downstream tasks.
However, at some point further model increases become harder due to GPU/TPU memory limitations, longer training times, and unexpected model degradation.
To address these problems,  we present two parameter-reduction techniques to lower memory consumption and increase the training speed of BERT.
Comprehensive empirical evidence shows that our proposed methods lead to models that scale much better compared to the original BERT.
We also use a self-supervised loss that focuses on modeling inter-sentence coherence, and show it consistently helps downstream tasks with multi-sentence inputs.
As a result, our best model establishes new state-of-the-art results on the GLUE, RACE, and SQuAD benchmarks while having fewer parameters compared to BERT-large.
Full network pre-training (Dai & Le, 2015; Radford et al., 2018; Howard & Ruder, 2018 ) has led to a series of breakthroughs in language representation learning.
Many nontrivial NLP tasks, including those that have limited training data, have greatly benefited from these pre-trained models.
One of the most compelling signs of these breakthroughs is the evolution of machine performance on a reading comprehension task designed for middle and high-school English exams in China, the RACE test (Lai et al., 2017) : the paper that originally describes the task and formulates the modeling challenge reports then state-of-the-art machine accuracy at 44.1%; the latest published result reports their model performance at 83.2% ; the work we present here pushes it even higher to 89.4%, a stunning 45.3% improvement that is mainly attributable to our current ability to build high-performance pretrained language representations.
Evidence from these improvements reveals that a large network is of crucial importance for achieving state-of-the-art performance .
It has become common practice to pre-train large models and distill them down to smaller ones (Sun et al., 2019; Turc et al., 2019) for real applications.
Given the importance of model size, we ask: Is having better NLP models as easy as having larger models?
An obstacle to answering this question is the memory limitations of available hardware.
Given that current state-of-the-art models often have hundreds of millions or even billions of parameters, it is easy to hit these limitations as we try to scale our models.
Training speed can also be significantly hampered in distributed training, as the communication overhead is directly proportional to the number of parameters in the model.
We also observe that simply growing the hidden size of a model such as BERT-large can lead to worse performance.
Table 1 and Fig. 1 show a typical example, where we simply increase the hidden size of BERT-large to be 2x larger and get worse results with this BERT-xlarge model.
Model Hidden Size Parameters RACE (Accuracy) BERT-large 1024 334M 72.0% BERT-large (ours) 1024 334M 73.9% BERT-xlarge (ours) 2048 1270M 54.3% Table 1 : Increasing hidden size of BERT-large leads to worse performance on RACE.
Existing solutions to the aforementioned problems include model parallelization (Shoeybi et al., 2019) and clever memory management (Chen et al., 2016; .
These solutions address the memory limitation problem, but not the communication overhead and model degradation problem.
In this paper, we address all of the aforementioned problems, by designing A Lite BERT (ALBERT) architecture that has significantly fewer parameters than a traditional BERT architecture.
ALBERT incorporates two parameter reduction techniques that lift the major obstacles in scaling pre-trained models.
The first one is a factorized embedding parameterization.
By decomposing the large vocabulary embedding matrix into two small matrices, we separate the size of the hidden layers from the size of vocabulary embedding.
This separation makes it easier to grow the hidden size without significantly increasing the parameter size of the vocabulary embeddings.
The second technique is cross-layer parameter sharing.
This technique prevents the parameter from growing with the depth of the network.
Both techniques significantly reduce the number of parameters for BERT without seriously hurting performance, thus improving parameter-efficiency.
An ALBERT configuration similar to BERT-large has 18x fewer parameters and can be trained about 1.7x faster.
The parameter reduction techniques also act as a form of regularization that stabilizes the training and helps with generalization.
To further improve the performance of ALBERT, we also introduce a self-supervised loss for sentence-order prediction (SOP).
SOP primary focuses on inter-sentence coherence and is designed to address the ineffectiveness of the next sentence prediction (NSP) loss proposed in the original BERT.
As a result of these design decisions, we are able to scale up to much larger ALBERT configurations that still have fewer parameters than BERT-large but achieve significantly better performance.
We establish new state-of-the-art results on the well-known GLUE, SQuAD, and RACE benchmarks for natural language understanding.
Specifically, we push the RACE accuracy to 89.4%, the GLUE benchmark to 89.4, and the F1 score of SQuAD 2.0 to 92.2.
While ALBERT-xxlarge has less parameters than BERT-large and gets significantly better results, it is computationally more expensive due to its larger structure.
An important next step is thus to speed up the training and inference speed of ALBERT through methods like sparse attention and block attention (Shen et al., 2018 ).
An orthogonal line of research, which could provide additional representation power, includes hard example mining (Mikolov et al., 2013) efficient language modeling training .
Additionally, although we have convincing evidence that sentence order prediction is a more consistently-useful learning task that leads to better language representations, we hypothesize that there could be more dimensions not yet captured by the current self-supervised training losses that could create additional representation power for the resulting representations.
RACE RACE is a large-scale dataset for multi-choice reading comprehension, collected from English examinations in China with nearly 100,000 questions.
Each instance in RACE has 4 candidate answers.
Following prior work , we use the concatenation of the passage, question, and each candidate answer as the input to models.
Then, we use the representations from the "[CLS]" token for predicting the probability of each answer.
The dataset consists of two domains: middle school and high school.
We train our models on both domains and report accuracies on both the development set and test set.
A.2
HYPERPARAMETERS Hyperparameters for downstream tasks are shown in Table 15 .
We adapt these hyperparameters from , , and Yang et al. (2019

assistant: text: A new pretraining method that establishes new state-of-the-art results on the GLUE, RACE, and SQuAD benchmarks while having fewer parameters compared to BERT-large.