Towards Understanding the Universality of Transformers for Next-Token Prediction
Keywords: Transformers, In-Context Learning, Deep Learning Theory
TL;DR: We investigate how causal Transformers can predict the next token in a sequence by constructing models that use a causal kernel descent method to learn context-dependent functions.
Abstract: Causal Transformers are trained to predict the next token for a given context. While it is widely accepted that self-attention is crucial for encoding the causal structure of sequences, the precise underlying mechanism behind this in-context autoregressive learning ability remains unclear. In this paper, we take a step towards understanding this phenomenon by studying the approximation ability of Transformers for next-token prediction. Specifically, we explore the capacity of causal Transformers to predict the next token $x_{t+1}$ given an autoregressive sequence $(x_1, \dots, x_t)$ as a prompt, where $ x_{t+1} = f(x_t) $, and $ f $ is a context-dependent function that varies with each sequence.
On the theoretical side, we focus on specific instances, namely when $ f $ is linear or when $ (x_t)$ is periodic. We explicitly construct a Transformer (with linear, exponential, or softmax attention) that learns the mapping $f$ in-context through a causal kernel descent method. The causal kernel descent method we propose provably estimates $x_{t+1} $ based solely on past and current observations $ (x_1, \dots, x_t) $, with connections to the Kaczmarz algorithm in Hilbert spaces. We present experimental results that validate our theoretical findings and suggest their applicability to more general mappings $f$.
Primary Area: learning theory
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Submission Number: 6762
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