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Title: Feature Partitioning for Efficient Multi-Task Architectures

Paper content:
Multi-task learning promises to use less data, parameters, and time than training separate single-task models.
But realizing these benefits in practice is challenging.
In particular, it is difficult to define a suitable architecture that has enough capacity to support many tasks while not requiring excessive compute for each individual task.
There are difficult trade-offs when deciding how to allocate parameters and layers across a large set of tasks.
To address this, we propose a method for automatically searching over multi-task architectures that accounts for resource constraints.
We define a parameterization of feature sharing strategies for effective coverage and sampling of architectures.
We also present a method for quick evaluation of such architectures with feature distillation.
Together these contributions allow us to quickly optimize for parameter-efficient multi-task models.
We benchmark on Visual Decathlon, demonstrating that we can automatically search for and identify architectures that effectively make trade-offs between task resource requirements while maintaining a high level of final performance.
Multi-task learning allows models to leverage similarities across tasks and avoid overfitting to the particular features of any one task (Caruana, 1997; Zamir et al., 2018) .
This can result in better generalization and more robust feature representations.
While this makes multi-task learning appealing for its potential performance improvements, there are also benefits in terms of resource efficiency.
Training a multi-task model should require less data, fewer training iterations, and fewer total parameters than training an equivalent set of task-specific models.
In this work we investigate how to automatically search over high performing multi-task architectures while taking such resource constraints into account.
Finding architectures that offer the best accuracy possible given particular resource constraints is nontrivial.
There are subtle trade-offs in performance when increasing or reducing use of parameters and operations.
Furthermore, with multiple tasks, one must take into account the impact of shared operations.
There is a large space of options for tweaking such architectures, in fact so large that it is difficult to tune an optimal configuration manually.
Neural architecture search (NAS) allows researchers to automatically search for models that offer the best performance trade-offs relative to some metric of efficiency.
Here we define a multi-task architecture as a single network that supports separate outputs for multiple tasks.
These outputs are produced by unique execution paths through the model.
In a neural network, such a path is made up of a subset of the total nodes and operations in the model.
This subset may or may not overlap with those of other tasks.
During inference, unused parts of the network can be ignored by either pruning out nodes or zeroing out their activations (Figure 1 ).
Such architectures mean improved parameter efficiency because redundant operations and features can be consolidated and shared across a set of tasks.
We seek to optimize for the computational efficiency of multi-task architectures by finding models that perform as well as possible while reducing average node use per task.
Different tasks will require different capacities to do well, so reducing average use requires effectively identifying which tasks will ask more of the model and which tasks can perform well with less.
In addition, performance is affected by how nodes are shared across tasks.
It is unclear when allocating resources whether sets of tasks would benefit from sharing parameters or would instead interfere.
Figure 1: Feature partitioning can be used to control how much network capacity is used by tasks, and how much sharing is done across tasks.
In this work we identify effective partitioning strategies to maximize performance while reducing average computation per task.
When searching over architectures, differences in resource use can be compared at different levels of granularity.
Most existing work in NAS and multi-task learning searches over the allocation and use of entire layers (Zoph & Le, 2016; Fernando et al., 2017; Rosenbaum et al., 2017) , we instead partition out individual feature channels within a layer.
This offers a greater degree of control over both the computation required by each task and the sharing that takes place between tasks.
The main obstacle to address in searching for effective multi-task architectures is the vast number of possibilities for performing feature partitioning as well as the significant amount of computation required to evaluate and compare arrangements.
A naive brute search over different partitioning strategies is prohibitively expensive.
We leverage our knowledge of the search space to explore it more effectively.
We propose a parameterization of partitioning strategies to reduce the size of the search space by eliminating unnecessary redundancies and more compactly expressing the key features that distinguish different architectures.
In addition, the main source of overhead in NAS is evaluation of sampled architectures.
It is common to define a surrogate operation that can be used in place of training a full model to convergence.
Often a smaller model will be trained for a much shorter number of iterations with the hope that the differences in accuracy that emerge early on correlate with the final performance of the full model.
We propose a strategy for evaluating multi-task architectures using feature distillation which provides much faster feedback on the effectiveness of a proposed partitioning strategy while correlating well with final validation accuracy.
In this work we provide:
• a parameterization that aids automatic architecture search by providing a direct and compact representation of the space of sharing strategies in multi-task architectures.
• an efficient method for evaluating proposed parameterizations using feature distillation to further accelerate the search process.
• results on Visual Decathlon (Rebuffi et al., 2017) to demonstrate that our search strategy allows us to effectively identify trade-offs between parameter use and performance on diverse and challenging image classification datasets.
In this work we investigate efficient multi-task architecture search to quickly find models that achieve high performance under a limited per-task budget.
We propose a novel strategy for searching over feature partitioning that automatically determines how much network capacity should be used by each task and how many parameters should be shared between tasks.
We design a compact representation to serve as a search space, and show that we can quickly estimate the performance of different partitioning schemes by using feature distillation.

assistant: text: automatic search for multi-task architectures that reduce per-task feature use