Modular Meta Learning
CoRL 2018
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CoRL 2018
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Previous approaches to meta-learning have focused on finding distributions or initial values of parameters.
Our objective is similar, but rather than focusing on transferring information about parameter values, we focus on finding a set of reusable modules that can form components of a solution to a new task, possibly with a small amount of tuning.
The authors provide an algorithm, called BounceGrad, which learns a set of modules and then combines them appropriately for a new task.
Given the specification of a composition rule and a basis set of modules, represents a set of possible functional input-output mappings that will serve as the hypothesis space for the meta-test task.
is a basis set of modules, which are functions
Each function has a parametric form where is a fixed-dimensional vector of parameters.
In this work, all the are neural networks, potentially with different architectures, and the parameters are the weights of the neural networks, which differ among the modules. Some examples of modules:
Single module
A fixed compositional structure:
A weighted ensemble.
: the set of possible structures and is a particular structure generated by . This approach has two phases: an off-line meta-learning phase and an on-line meta-test learning phase.
Meta-learning phase: we take training and validation data sets for tasks as input and generate a parametrization for each module. The objective is to construct modules that will work together as good building blocks for future tasks.
At meta-learning time, is specified, and the objective is to find parameter values that constitute a set of modules that can be recombined to effectively solve each of the training tasks.
Validation set is used for the meta-training tasks to avoid choosing in a way that overfits.
The training objective is to find that minimizing the average generalization performance of the hypotheses using parameter set : (See above figure)
Meta-test learning phase: we take a training data set for the meta-test task as input, as well as and ; the output is a compositional form which includes a selection of modules to be used in that form. Sinceis already specified, the choice of completely determines a mapping from inputs to outputs.
It looks that this is a bi-level optimization problem, and use iterative optimization method to solve it.
Notes:
Simulated Annealing
iteration optimization vs bilevel optimization