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Title: Towards Understanding Generalization in Gradient-Based Meta-Learning

Paper content:
In this work we study generalization of neural networks in gradient-based meta-learning by analyzing various properties of the objective landscapes.
We experimentally demonstrate that as meta-training progresses, the meta-test solutions obtained by adapting the meta-train solution of the model to new tasks via few steps of gradient-based fine-tuning, become flatter, lower in loss, and further away from the meta-train solution.
We also show that those meta-test solutions become flatter even as generalization starts to degrade, thus providing an experimental evidence against the correlation between generalization and flat minima in the paradigm of gradient-based meta-leaning.
Furthermore, we provide empirical evidence that generalization to new tasks is correlated with the coherence between their adaptation trajectories in parameter space, measured by the average cosine similarity between task-specific trajectory directions, starting from a same meta-train solution.
We also show that coherence of meta-test gradients, measured by the average inner product between the task-specific gradient vectors evaluated at meta-train solution, is also correlated with generalization.
To address the problem of the few-shot learning, many meta-learning approaches have been proposed recently (Finn et al., 2017) , (Ravi and Larochelle, 2017) , (Rothfuss et al., 2018) , (Oreshkin et al., 2018) and (Snell et al., 2017) among others.
In this work, we take steps towards understanding the characteristics of the landscapes of the loss functions, and their relation to generalization, in the context of gradient-based few-shot meta-learning.
While we are interested in understanding the properties of optimization landscapes that are linked to generalization in gradient-based meta-learning in general, we focus our experimental work here within a setup that follows the recently proposed Model Agnostic Meta-Learning (MAML) algorithm (Finn et al., 2017) .
The MAML algorithm is a good candidate for studying gradient-based meta-learning because of its independence from the underlying network architecture.
Our main insights and contributions can be summarized as follows:
1. As gradient-based meta-training progresses:
• the adapted meta-test solutions become flatter on average, while the opposite occurs when using a finetuning baseline.
• the adapted final solutions reach lower average support loss values, which never increases, while the opposite occurs when using a finetuning baseline.
2. When generalization starts to degrade due to overtraining, meta-test solutions keep getting flatter, implying that, in the context of gradient-based meta-learning, flatness of minima is not correlated with generalization to new tasks.
3. We empirically show that generalization to new tasks is correlated with the coherence between their adaptation trajectories, measured by the average cosine similarity between trajectory directions.
Also correlated with generalization is the coherence between metatest gradients, measured by the average inner product between meta-test gradient vectors evaluated at meta-train solution.
We also show that this metric is correlated to generalization for few-shot regression tasks where the model must learn to fit sine function curves.
Furthermore, based on these observations, we take initial steps to propose a regularizer for MAML based training and provide experimental evidence for its effectiveness.
We experimentally demonstrate that when using gradient-based meta-learning algorithms such as MAML, meta-test solutions, obtained after adapting neural networks to new tasks via few-shot learning, become flatter, lower in loss, and further away from the meta-train solution, as metatraining progresses.
We also show that those meta-test solutions keep getting flatter even when generalization starts to degrade, thus providing an experimental argument against the correlation between generalization and flat minima.
More importantly, we empirically show that generalization to new tasks is correlated with the coherence between their adaptation trajectories, measured by the average cosine similarity between the adaptation trajectory directions, but also correlated with the coherence between the meta-test gradients, measured by the average inner product between meta-test gradient vectors evaluated at meta-train solution.
We also show this correlation for few-shot regression tasks.
Based on these observations, we take first steps towards regularizing MAML based meta-training.
As a future work, we plan to test the effectiveness of this regularizer on various datasets and meta-learning problem settings, architectures and gradient-based meta-learning algorithms.
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assistant: text: We study generalization of neural networks in gradient-based meta- learning by analyzing various properties of the objective landscape.