The EOS Decision and Length Extrapolation
Benjamin Newman, John Hewitt, Percy Liang, Christopher D. Manning
Abstract
Extrapolation to unseen sequence lengths is a challenge for neural generative models of language. In this work, we characterize the effect on length extrapolation of a modeling decision often overlooked: predicting the end of the generative process through the use of a special end-of-sequence (EOS) vocabulary item. We study an oracle setting - forcing models to generate to the correct sequence length at test time - to compare the length-extrapolative behavior of networks trained to predict EOS (+EOS) with networks not trained to (-EOS). We find that -EOS substantially outperforms +EOS, for example extrapolating well to lengths 10 times longer than those seen at training time in a bracket closing task, as well as achieving a 40% improvement over +EOS in the difficult SCAN dataset length generalization task. By comparing the hidden states and dynamics of -EOS and +EOS models, we observe that +EOS models fail to generalize because they (1) unnecessarily stratify their hidden states by their linear position is a sequence (structures we call length manifolds) or (2) get stuck in clusters (which we refer to as length attractors) once the EOS token is the highest-probability prediction.- Anthology ID:
- 2020.blackboxnlp-1.26
- Volume:
- Proceedings of the Third BlackboxNLP Workshop on Analyzing and Interpreting Neural Networks for NLP
- Month:
- November
- Year:
- 2020
- Address:
- Online
- Editors:
- Afra Alishahi, Yonatan Belinkov, Grzegorz Chrupała, Dieuwke Hupkes, Yuval Pinter, Hassan Sajjad
- Venue:
- BlackboxNLP
- SIG:
- Publisher:
- Association for Computational Linguistics
- Note:
- Pages:
- 276–291
- Language:
- URL:
- https://aclanthology.org/2020.blackboxnlp-1.26
- DOI:
- 10.18653/v1/2020.blackboxnlp-1.26
- Cite (ACL):
- Benjamin Newman, John Hewitt, Percy Liang, and Christopher D. Manning. 2020. The EOS Decision and Length Extrapolation. In Proceedings of the Third BlackboxNLP Workshop on Analyzing and Interpreting Neural Networks for NLP, pages 276–291, Online. Association for Computational Linguistics.
- Cite (Informal):
- The EOS Decision and Length Extrapolation (Newman et al., BlackboxNLP 2020)
- PDF:
- https://preview.aclanthology.org/emnlp-22-attachments/2020.blackboxnlp-1.26.pdf
- Code
- bnewm0609/eos-decision
- Data
- SCAN