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Title: Learning to Balance: Bayesian Meta-Learning for Imbalanced and Out-of-distribution Tasks

Paper content:
While tasks could come with varying the number of instances and classes in realistic settings, the existing meta-learning approaches for few-shot classification assume that number of instances per task and class is fixed.
Due to such restriction, they learn to equally utilize the meta-knowledge across all the tasks, even when the number of instances per task and class largely varies.
Moreover, they do not consider distributional difference in unseen tasks, on which the meta-knowledge may have less usefulness depending on the task relatedness.
To overcome these limitations, we propose a novel meta-learning model that adaptively balances the effect of the meta-learning and task-specific learning within each task.
Through the learning of the balancing variables, we can decide whether to obtain a solution by relying on the meta-knowledge or task-specific learning.
We formulate this objective into a Bayesian inference framework and tackle it using variational inference.
We validate our Bayesian Task-Adaptive Meta-Learning (Bayesian TAML) on two realistic task- and class-imbalanced datasets, on which it significantly outperforms existing meta-learning approaches.
Further ablation study confirms the effectiveness of each balancing component and the Bayesian learning framework.
Despite the success of deep learning in many real-world tasks such as visual recognition and machine translation, such good performances are achievable at the availability of large training data, and many fail to generalize well in small data regimes.
To overcome this limitation of conventional deep learning, recently, researchers have explored meta-learning (Schmidhuber, 1987; Thrun & Pratt, 1998) approaches, whose goal is to learn a model that generalizes well over distribution of tasks, rather than instances from a single task, in order to utilize the obtained meta-knowledge across tasks to compensate for the lack of training data for each task.
However, so far, most existing meta-learning approaches (Santoro et al., 2016; Vinyals et al., 2016; Snell et al., 2017; Ravi & Larochelle, 2017; Finn et al., 2017; have only targeted an artificial scenario where all tasks participating in the multi-class classification problem have equal number of training instances per class.
Yet, this is a highly restrictive setting, as in real-world scenarios, tasks that arrive at the model may have different training instances (task imbalance), and within each task, the number of training instances per class may largely vary (class imbalance).
Moreover, the new task may come from a distribution that is different from the task distribution the model has been trained on (out-of-distribution task) (See (a) of Figure 1 ).
Under such a realistic setting, the meta-knowledge may have a varying degree of utility to each task.
Tasks with small number of training data, or close to the tasks trained in meta-training step may want to rely mostly on meta-knowledge obtained over other tasks, whereas tasks that are out-of-distribution or come with more number of training data may obtain better solutions when trained in a task-specific manner.
Furthermore, for multi-class classification, we may want to treat the learning for each class differently to handle class imbalance.
Thus, to optimally leverage meta-learning under various imbalances, it would be beneficial for the model to task-and class-adaptively decide how much to use from the meta-learner, and how much to learn specifically for each task and class.
We propose Bayesian TAML that learns to balance the effect of meta-learning and task-adaptive learning, to consider meta-learning under a more realistic task distribution where each task and class can have varying number of instances.
Specifically, we encode the dataset for each task into hierarchical set-of-sets representations, and use it to generate attention mask for the original parameter, learning rate decay, and the class-specific learning rate.
We use a Bayesian framework to infer the posterior of these balancing variables, and propose an effective variational inference framework to solve for them.
Our model outperforms existing meta-learning methods when validated on imbalanced few-shot classification tasks.
Further analysis of each balancing variable shows that each variable effectively handles task imbalance, class imbalance, and out-of-distribution tasks respectively.
We believe that our work makes a meaningful step toward application of meta-learning to real-world problems.
A EXPERIMENTAL SETUP A.1
BASELINES AND NETWORK ARCHITECTURE.
We describe baseline models and our task-adaptive learning to balance model.
Note that all gradientbased models can be extended to take K inner-gradient steps for both meta-training and meta-testing.
1) Meta-Learner LSTM.
A meta-learner that learns optimization algorithm with LSTM (Ravi & Larochelle, 2017) .
The model performs few-shot classification using cosine similarities between the embeddings generated from a shared convolutional network.
2) Prototypical Networks.
A metric-based few-shot classification model proposed by (Snell et al., 2017) .
The model learns the metric space based on Euclidean distance between class prototypes and query embeddings.
3) MAML.
The Model-Agnostic Meta-Learning (MAML) model by (Finn et al., 2017) , which aims to learn the global initial model parameter, from which we can take a few gradient steps to get task-specific predictors.

assistant: text: A novel meta-learning model that adaptively balances the effect of the meta-learning and task-specific learning, and also class-specific learning within each task.