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Title: Rapid Learning or Feature Reuse? Towards Understanding the Effectiveness of MAML

Paper content:
An important research direction in machine learning has centered around developing meta-learning algorithms to tackle few-shot learning.
An especially successful algorithm has been Model Agnostic Meta-Learning (MAML), a method that consists of two optimization loops, with the outer loop finding a meta-initialization, from which the inner loop can efficiently learn new tasks.
Despite MAML's popularity, a fundamental open question remains -- is the effectiveness of MAML due to the meta-initialization being primed for rapid learning (large, efficient changes in the representations) or due to feature reuse,  with the meta initialization already containing high quality features?
We investigate this question, via ablation studies and analysis of the latent representations, finding that feature reuse is the dominant factor.
This leads to the ANIL (Almost No Inner Loop) algorithm, a simplification of MAML where we remove the inner loop for all but the (task-specific) head of the underlying neural network.
ANIL matches MAML's performance on benchmark few-shot image classification and RL and offers computational improvements over MAML.
We further study the precise contributions of the head and body of the network, showing that performance on the test tasks is entirely determined by the quality of the learned features, and we can remove even the head of the network (the NIL algorithm).
We conclude with a discussion of the rapid learning vs feature reuse question for meta-learning algorithms more broadly.
A central problem in machine learning is few-shot learning, where new tasks must be learned with a very limited number of labelled datapoints.
A significant body of work has looked at tackling this challenge using meta-learning approaches (16; 37; 32; 6; 30; 28; 24) .
Broadly speaking, these approaches define a family of tasks, some of which are used for training and others solely for evaluation.
A proposed meta-learning algorithm then looks at learning properties that generalize across the different training tasks, and result in fast and efficient learning of the evaluation tasks.
One highly successful meta-learning algorithm has been Model Agnostic Meta-Learning (MAML) (6) .
At a high level, the MAML algorithm is comprised of two optimization loops.
The outer loop (in the spirit of meta-learning) aims to find an effective meta-initialization, from which the inner loop can perform efficient adaptation -optimize parameters to solve new tasks with very few labelled examples.
This algorithm, with deep neural networks as the underlying model, has been highly influential, with significant follow on work, such as first order variants (24) , probabilistic extensions (8) , augmentation with generative modelling (29) , and many others (15; 7; 12; 35) .
Despite the popularity of MAML, and the numerous followups and extensions, there remains a fundamental open question on the basic algorithm.
Does the meta-initialization learned by the outer loop result in rapid learning on unseen test tasks (efficient but significant changes in the representations) or is the success primarily due to feature reuse (with the meta-initialization already providing high quality representations)?
In this paper, we explore this question and its many surprising consequences.
Our main contributions are:
• We perform layer freezing experiments and latent representational analysis of MAML, finding that feature reuse is the predominant reason for efficient learning.
• Based on these results, we propose the ANIL (Almost No Inner Loop) algorithm, a significant simplification to MAML that removes the inner loop updates for all but the head (final layer) of a neural network during training and inference.
ANIL performs identically to MAML on standard benchmark few-shot classification and RL tasks and offers computational benefits over MAML.
• We study the effect of the head of the network, finding that once training is complete, the head can be removed, and the representations can be used without adaptation to perform unseen tasks, which we call the No Inner Loop (NIL) algorithm.
• We study different training regimes, e.g. multiclass classification, multitask learning, etc, and find that the task specificity of MAML/ANIL at training facilitate the learning of better features.
We also find that multitask training, a popular baseline with no task specificity, performs worse than random features.
• We discuss rapid learning and feature reuse in the context of other meta-learning approaches.
In this paper, we studied a fundamental question on whether the highly successful MAML algorithm relies on rapid learning or feature reuse.
Through a series of experiments, we found that feature reuse is the dominant component in MAML's efficacy.
This insight led to the ANIL (Almost No Inner Loop) algorithm, a simplification of MAML that has identical performance on standard image classification and reinforcement learning benchmarks, and provides computational benefits.
We further study the importance of the head (final layer) of a neural network trained with MAML, discovering that the body (lower layers) of a network is sufficient for few-shot classification at test time, allowing us to remove the network head for testing (NIL) and still match performance.
We connected our results to the broader literature in meta-learning, identifying feature reuse to be a common mode of operation for other meta-learning algorithms also.
Based off of our conclusions, future work could look at developing and analyzing new meta-learning algorithms that perform more rapid learning, which may expand the datasets and problems amenable to these techniques.

assistant: text: The success of MAML relies on feature reuse from the meta-initialization, which also yields a natural simplification of the algorithm, with the inner loop removed for the network body, as well as other insights on the head and body.