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Title: Meta-Learning with Warped Gradient Descent

Paper content:
Learning an efficient update rule from data that promotes rapid learning of new tasks from the same distribution remains an open problem in meta-learning.
Typically, previous works have approached this issue either by attempting to train a neural network that directly produces updates or by attempting to learn better initialisations or scaling factors for a gradient-based update rule.
Both these approaches pose challenges.
On one hand, directly producing an update forgoes a useful inductive bias and can easily lead to non-converging behaviour.
On the other hand, approaches that try to control a gradient-based update rule typically resort to computing gradients through the learning process to obtain their meta-gradients, leading to methods that can not scale beyond few-shot task adaptation.
In this work we propose Warped Gradient Descent (WarpGrad), a method that intersects these approaches to mitigate their limitations.
WarpGrad meta-learns an efficiently parameterised preconditioning matrix that facilitates gradient descent across the task distribution.
Preconditioning arises by interleaving non-linear layers, referred to as warp-layers, between the layers of a task-learner.
Warp-layers are meta-learned without backpropagating through the task training process in a manner similar to methods that learn to directly produce updates.
WarpGrad is computationally efficient, easy to implement, and can scale to arbitrarily large meta-learning problems.
We provide a geometrical interpretation of the approach and evaluate its effectiveness in a variety of settings, including few-shot, standard supervised, continual and reinforcement learning.
to learn implies inferring a learning strategy from some set of past experiences via a meta-learner that a task-learner can leverage when learning a new task.
One approach is to directly parameterise an update rule via the memory of a recurrent neural network (Andrychowicz et al., 2016; Ravi & Larochelle, 2016; Li & Malik, 2016; Chen et al., 2017) .
Such memory-based methods can, in principle, represent any learning rule by virtue of being universal function approximators (Cybenko, 1989; Hornik, 1991; Schäfer & Zimmermann, 2007) .
They can also scale to long learning processes by using truncated backpropagation through time, but they lack an inductive bias as to what constitutes a reasonable learning rule.
This renders them hard to train and brittle to generalisation as their parameter updates have no guarantees of convergence.
An alternative family of approaches defines a gradient-based update rule and meta-learns a shared initialisation that facilitates task adaptation across a distribution of tasks (Finn et al., 2017; Nichol et al., 2018; Flennerhag et al., 2019) .
Such methods are imbued with a strong inductive biasgradient descent-but restrict knowledge transfer to the initialisation.
Recent work has shown that it is beneficial to more directly control gradient descent by meta-learning an approximation of a parameterised matrix (Li et al., 2017; Lee et al., 2017; Park & Oliva, 2019 ) that preconditions gradients during task training, similarly to second-order and Natural Gradient Descent methods (Nocedal & Wright, 2006; Amari & Nagaoka, 2007) .
To meta-learn preconditioning, these methods backpropagate through the gradient descent process, limiting them to few-shot learning.
In this paper, we propose a novel framework called Warped Gradient Descent (WarpGrad) , that relies on the inductive bias of gradient-based meta-learners by defining an update rule that preconditions gradients, but that is meta-learned using insights from memory-based methods.
In particular, we
We propose WarpGrad, a novel meta-learner that combines the expressive capacity and flexibility of memory-based meta-learners with the inductive bias of gradient-based meta-learners.
WarpGrad meta-learns to precondition gradients during task adaptation without backpropagating through the adaptation process and we find empirically that it retains the inductive bias of MAML-based few-shot learners while being able to scale to complex problems and architectures.
Further, by expressing preconditioning through warp-layers that are universal function approximators, WarpGrad is able to express geometries beyond the block-diagonal structure of prior works.
WarpGrad provides a principled framework for general-purpose meta-learning that integrates learning paradigms, such as continual learning, an exciting avenue for future research.
We introduce novel means for preconditioning, for instance with residual and recurrent warp-layers.
Understanding how WarpGrad manifolds relate to second-order optimisation methods will further our understanding of gradient-based meta-learning and aid us in designing warp-layers with stronger inductive bias.
In their current form, WarpGrad methods share some of the limitations of many popular metalearning approaches.
While WarpGrad is designed to avoid backpropagating through the task training process, as in Warp-Leap, the WarpGrad objective samples from parameter trajectories and has therefore linear computational complexity in the number of adaptation steps, currently an unresolved limitation of gradient-based meta-learning.
Our offline algorithm (Algorithm 2) hints at exciting possibilities for overcoming this limitation.
WarpGrad is a model-embedded meta-learned optimiser that allows for a number of implementation strategies.
Indeed, there is a number of ways warp-layers can be embedded in an architecture of choice.
To embed warp-layers given a task-learner architecture, we may either insert new warp-layers in the given architecture or designate some layers as warp-layers and some as task layers.
We found that WarpGrad can both be used in a high-capacity mode, where task-learners are relatively weak to avoid overfitting, as well as in a low-capacity mode where task-learners are powerful and warp-layers are relatively weak.
The best approach depends on the problem at hand.
We highlight three approaches to designing WarpGrad optimisers, starting from a given architecture:
(a) Model partitioning.
Given a desired architecture, designate some operations as task-adaptable and the rest as warp-layers.
Task layers do not have to interleave exactly with warp-layers as gradient warping arises both through the forward pass and through backpropagation.
This was how we approached the tieredImageNet and miniImageNet experiments.
(b) Model augmentation.
Given a model, designate all layers as task-adaptable and interleave warplayers.
Warp-layers can be relatively weak as backpropagation through non-linear activations ensures expressive gradient warping.
This was our approach to the Omniglot experiment; our main architecture interleaves linear warp-layers in a standard architecture.
(c) Information compression.
Given a model, designate all layers as warp and interleave weak task layers.
In this scenario, task-learners are prone to overfitting.
Pushing capacity into the warp allows it to encode general information the task-learner can draw on during task adaptation.
This approach is similar to approaches in transfer and meta-learning that restrict the number of free parameters during task training (Rebuffi et al., 2017; Lee & Choi, 2018; Zintgraf et al., 2019 Figure 6 illustrates this process.

assistant: text: We propose a novel framework for meta-learning a gradient-based update rule that scales to beyond few-shot learning and is applicable to any form of learning, including continual learning.