Erik Nijkamp


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Long Document Summarization with Top-down and Bottom-up Inference
Bo Pang | Erik Nijkamp | Wojciech Kryscinski | Silvio Savarese | Yingbo Zhou | Caiming Xiong
Findings of the Association for Computational Linguistics: EACL 2023

Text summarization aims to condense long documents and retain key information. Critical to the success of a summarization model is the faithful inference of latent representations of words or tokens in the source documents. Most recent models infer the latent representations with a transformer encoder, which is purely bottom-up and thus does not capture long-distance context well. Also, self-attention-based models face the challenge of quadratic complexity with respect to sequence length. We propose a method to improve summarization models on these two aspects. Our method assumes a hierarchical latent structure of a document where the top-level captures the long range dependency at a coarser time scale and the bottom token level preserves the details. Critically, our method enables token representations to be updated in both a bottom-up and top-down manner. In the bottom-up pass, token representations are inferred with local self-attention to leverage its efficiency. Top-down correction is then applied to allow tokens to capture global context. We demonstrate the effectiveness on a diverse set of summarization datasets, including narrative, conversational, scientific documents and news. Our model achieves state-of-the-art performance on a wide range of long document summarization benchmarks, compared to recent efficient transformers. We show that our model can summarize an entire book and achieve competitive performance using 0.27% parameters and much less training data, compared to a recent GPT-3-based model. These results indicate the general applicability and benefits of the framework.


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SCRIPT: Self-Critic PreTraining of Transformers
Erik Nijkamp | Bo Pang | Ying Nian Wu | Caiming Xiong
Proceedings of the 2021 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies

We introduce Self-CRItic Pretraining Transformers (SCRIPT) for representation learning of text. The popular masked language modeling (MLM) pretraining methods like BERT replace some tokens with [MASK] and an encoder is trained to recover them, while ELECTRA trains a discriminator to detect replaced tokens proposed by a generator. In contrast, we train a language model as in MLM and further derive a discriminator or critic on top of the encoder without using any additional parameters. That is, the model itself is a critic. SCRIPT combines MLM training and discriminative training for learning rich representations and compute- and sample-efficiency. We demonstrate improved sample-efficiency in pretraining and enhanced representations evidenced by improved downstream task performance on GLUE and SQuAD over strong baselines. Also, the self-critic scores can be directly used as pseudo-log-likelihood for efficient scoring.

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Generative Text Modeling through Short Run Inference
Bo Pang | Erik Nijkamp | Tian Han | Ying Nian Wu
Proceedings of the 16th Conference of the European Chapter of the Association for Computational Linguistics: Main Volume

Latent variable models for text, when trained successfully, accurately model the data distribution and capture global semantic and syntactic features of sentences. The prominent approach to train such models is variational autoencoders (VAE). It is nevertheless challenging to train and often results in a trivial local optimum where the latent variable is ignored and its posterior collapses into the prior, an issue known as posterior collapse. Various techniques have been proposed to mitigate this issue. Most of them focus on improving the inference model to yield latent codes of higher quality. The present work proposes a short run dynamics for inference. It is initialized from the prior distribution of the latent variable and then runs a small number (e.g., 20) of Langevin dynamics steps guided by its posterior distribution. The major advantage of our method is that it does not require a separate inference model or assume simple geometry of the posterior distribution, thus rendering an automatic, natural and flexible inference engine. We show that the models trained with short run dynamics more accurately model the data, compared to strong language model and VAE baselines, and exhibit no sign of posterior collapse. Analyses of the latent space show that interpolation in the latent space is able to generate coherent sentences with smooth transition and demonstrate improved classification over strong baselines with latent features from unsupervised pretraining. These results together expose a well-structured latent space of our generative model.


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Towards Holistic and Automatic Evaluation of Open-Domain Dialogue Generation
Bo Pang | Erik Nijkamp | Wenjuan Han | Linqi Zhou | Yixian Liu | Kewei Tu
Proceedings of the 58th Annual Meeting of the Association for Computational Linguistics

Open-domain dialogue generation has gained increasing attention in Natural Language Processing. Its evaluation requires a holistic means. Human ratings are deemed as the gold standard. As human evaluation is inefficient and costly, an automated substitute is highly desirable. In this paper, we propose holistic evaluation metrics that capture different aspects of open-domain dialogues. Our metrics consist of (1) GPT-2 based context coherence between sentences in a dialogue, (2) GPT-2 based fluency in phrasing, (3) n-gram based diversity in responses to augmented queries, and (4) textual-entailment-inference based logical self-consistency. The empirical validity of our metrics is demonstrated by strong correlations with human judgments. We open source the code and relevant materials.