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Levenshtein Transformer (1905.11006v2)

Published 27 May 2019 in cs.CL and cs.LG

Abstract: Modern neural sequence generation models are built to either generate tokens step-by-step from scratch or (iteratively) modify a sequence of tokens bounded by a fixed length. In this work, we develop Levenshtein Transformer, a new partially autoregressive model devised for more flexible and amenable sequence generation. Unlike previous approaches, the atomic operations of our model are insertion and deletion. The combination of them facilitates not only generation but also sequence refinement allowing dynamic length changes. We also propose a set of new training techniques dedicated at them, effectively exploiting one as the other's learning signal thanks to their complementary nature. Experiments applying the proposed model achieve comparable performance but much-improved efficiency on both generation (e.g. machine translation, text summarization) and refinement tasks (e.g. automatic post-editing). We further confirm the flexibility of our model by showing a Levenshtein Transformer trained by machine translation can straightforwardly be used for automatic post-editing.

Citations (349)

Summary

  • The paper presents the Levenshtein Transformer, which uses dynamic insertion and deletion operations to enhance sequence generation flexibility.
  • Its dual policy learning methodology boosts efficiency, achieving up to five times faster processing while delivering superior output quality.
  • Experimental results validate its effectiveness in machine translation and post-editing, demonstrating significant adaptability and robustness.

An Analysis of the Levenshtein Transformer for Flexible Sequence Generation

The paper "Levenshtein Transformer" introduces an innovative approach for sequence generation tasks through the application of insertion and deletion operations, diverging from conventional autoregressive or non-autoregressive models that predominately rely on either generating sequences token by token or through fixed-length structures. The Levenshtein Transformer (LevT) proposes a partially autoregressive sequence generation approach, specifically designed to enhance predictive flexibility and model efficiency. This model is distinguished by its ability to enable dynamic length adjustments and simultaneous sequence refinement, integrating insertion and deletion mechanisms to modify sequences.

Methodology

The Levenshtein Transformer operates by imitating human intelligence's nonlinear approach, where elements in a sequence can be easily added, removed, or modified. This model employs the imitation learning framework, leveraging dual-policy learning consisting of two interchangeable policies for insertion and deletion, with each operation guided by the output of its complementing adversary policy, effectively providing corrective signals. Dual policy learning allows the system to take advantage of the complementary nature of the insertion and deletion operations, thus enhancing learning robustness and efficiency.

In terms of training, the authors apply techniques that exploit the adversarial properties of the insertion and deletion operations, implementing a strategy called dual policy learning. This involves using the complement to provide input, while an expert policy acts as a guide for adjustments.

Experimental Results and Performance

Empirical evaluations demonstrate that the Levenshtein Transformer outperforms traditional Transformer models in both efficiency and quality across tasks such as machine translation and text summarization. Specifically, LevT achieves execution speed-ups up to five times while maintaining or even exceeding the quality of the outputs generated by strong Transformer baselines. Notably, experiments showed that the LevT can handle language tasks with significant consistency and a reduced number of iterations.

The paper also explores the flexibility of LevT for sequence refinement tasks such as automatic post-editing (APE), highlighting LevT's ability to be directly applied in zero-shot post-editing settings, an aspect unattainable by most current frameworks due to their architectural constraints that prevent intertwining generation and refinement.

Implications and Future Directions

From a theoretical standpoint, the LevT has expanded the boundaries of sequence prediction models by offering a unified framework where both sequence generation and refinement are treated under the same model architecture. This dual-function capability enhances the utility of LevT in scenarios requiring adjustments to pre-generated sequences, making it particularly suitable for APE tasks.

Looking ahead, further investigation into human interaction loops with AI models where dynamic adjustments like LevT could be beneficial is warranted. This approach opens new avenues for research in edit-based sequence generation systems, where direct human input might enhance model guidance, ultimately refining output quality and relevance.

In conclusion, the Levenshtein Transformer sets a new standard in sequence generation flexibility and efficiency, posing significant potential for adoption and adaptation across diverse AI applications requiring nuanced understanding and manipulation of sequences. This model's adaptability for different tasks without retraining reiterates its robust nature and potential to innovate AI-driven textual generation and refinement processes.

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