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Learning to (Learn at Test Time)

Paper · 2023 · Yu Sun, Xinhao Li, Karan Dalal et al.

Summary

Test-time training (TTT) adapts a model to each individual input through a self-supervised objective. Until now, picking an auxiliary task whose improvement actually helps the main task has depended on manual design and trial and error. Sun et al. recast supervised learning as a two-level meta-learning problem, which they call MTTT. The inner loop runs a few gradient steps of token reconstruction on one instance, treating that instance's tokens as a small dataset. The outer loop then trains the components of this reconstruction setup so that predictions made after adaptation improve. These components are the encoder initialization, the decoder, the input transforms and the prediction head. The authors show that a linear inner learner reduces this procedure to linear attention, while a kernel estimator reproduces softmax self-attention. TTT layers can therefore stand in for attention layers inside a transformer. With small MLPs as inner learners, the model outperforms linear-attention transformers on ImageNet from raw 224×224 pixels, in both accuracy and FLOPs, a setting where standard transformers run out of memory. The work presents TTT as a principled and scalable basis for designing sequence architectures.

Key points

  • Linear case: a linear encoder initialized at W₀ = 0 and updated by one gradient step with learning rate 1 is mathematically identical to linear attention. The roles of key, query and value weights are taken by the input-transform and decoder parameters.
  • Kernel case: a Nadaraya-Watson estimator with an exponential kernel matches softmax self-attention. Multi-head attention corresponds to H parallel inner loops, each with (d/H)×(d/H) weights.
  • MLP case: the variant with an MLP as the inner learner (MTTT-MLP) uses 12 TTT layers and T = 4 inner steps. Its inner loop runs full-batch gradient descent on ImageNet patches and stochastic gradient descent on raw pixels.
  • Extra tricks: output normalization (an additional LayerNorm on the reconstruction) and inner-loop SGD further improve results.
  • Implementation: the system is written in JAX, with gradients propagated through the inner-loop updates. Code is available at github.com/test-time-training/mttt.

Our summary of the paper, in our own words — not its text. The work belongs to its authors; read it at the links above.

 Institutions

 Fields, topics and concepts

Cite

Cite as Yu Sun, Xinhao Li, Karan Dalal, Chloe Hsu, Sanmi Koyejo, Carlos Guestrin, Xiaolong Wang, Tatsunori Hashimoto and Xinlei Chen. 2023. Learning to (Learn at Test Time). arXiv:2310.13807.

The paper itself, not this page. BibTeX from the library's record of the work. DOI 10.48550/arxiv.2310.13807 · arXiv 2310.13807

@misc{sun2023learning,
  title = {{Learning to (Learn at Test Time)}},
  author = {Yu Sun, Xinhao Li, Karan Dalal, Chloe Hsu, Sanmi Koyejo, Carlos Guestrin, Xiaolong Wang, Tatsunori Hashimoto and Xinlei Chen},
  year = {2023},
  doi = {10.48550/arxiv.2310.13807},
  eprint = {2310.13807},
  archivePrefix = {arXiv},
  url = {https://doi.org/10.48550/arxiv.2310.13807}
}

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