ALiBi - Train Short, Test Long: Attention with linear biases enables input length extrapolation

#alibi #transformers #attention

Transformers are essentially set models that need additional inputs to make sense of sequence data. The most widespread additional inputs are position encodings or position embeddings, which add sequence index information in various forms. However, this has put a limit on the resulting model, which cannot run inference on sequences longer than it has been trained on, as it would encounter unfamiliar position encodings. ALiBi solves this by proposing simple linear fixed biases as position information, adding negligible overhead in time and memory, but surprisingly, the resulting model is able to handle inference on sequences many times as long as its training sequences.

OUTLINE:
0:00 - Intro & Overview
1:40 - Position Encodings in Transformers
4:55 - Sinusoidial Position Encodings
11:50 - ALiBi Position Encodings
20:50 - How to choose the slope parameter
23:55 - Experimental Results
29:10 - Comments & Conclusion

Paper: https://ofir.io/train_short_test_long...
Code: https://github.com/ofirpress/attentio...

Abstract:
Since the introduction of the transformer model by Vaswani et al. (2017), a fundamental question remains open: how to achieve extrapolation at inference time to longer sequences than seen during training? We first show that extrapolation can be improved by changing the position representation method, though we find that existing proposals do not allow efficient extrapolation. We introduce a simple and efficient method, Attention with Linear Biases (ALiBi), that allows for extrapolation. ALiBi does not add positional embeddings to the word embeddings; instead, it biases the query-key attention scores with a term that is proportional to their distance. We show that this method allows training a 1.3 billion parameter model on input sequences of length 1024 that extrapolates to input sequences of length 2048, achieving the same perplexity as a sinusoidal position embedding model trained on inputs of length 2048, 11% faster and using 11% less memory. ALiBi’s inductive bias towards recency allows it to outperform multiple strong position methods on the WikiText-103 benchmark. Finally, we provide analysis of ALiBi to understand why it leads to better performance.

Authors: Ofir Press, Noah A. Smith, Mike Lewis

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