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Title: Lifelong Learning with Dynamically Expandable Networks

Paper content:
We propose a novel deep network architecture for lifelong learning which we refer to as Dynamically Expandable Network (DEN), that can dynamically decide its network capacity as it trains on a sequence of tasks, to learn a compact overlapping knowledge sharing structure among tasks.
DEN is efficiently trained in an online manner by performing selective retraining, dynamically expands network capacity upon arrival of each task with only the necessary number of units, and effectively prevents semantic drift by splitting/duplicating units and timestamping them.
We validate DEN on multiple public datasets in lifelong learning scenarios on multiple public datasets, on which it not only significantly outperforms existing lifelong learning methods for deep networks, but also achieves the same level of performance as the batch model with substantially fewer number of parameters.
Lifelong learning BID13 , the problem of continual learning where tasks arrive in sequence, is an important topic in transfer learning.
The primary goal of lifelong learning is to leverage knowledge from earlier tasks for obtaining better performance, or faster convergence/training speed on models for later tasks.
While there exist many different approaches to tackle this problem, we consider lifelong learning under deep learning to exploit the power of deep neural networks.
Fortunately, for deep learning, storing and transferring knowledge can be done in a straightforward manner through the learned network weights.
The learned weights can serve as the knowledge for the existing tasks, and the new task can leverage this by simply sharing these weights.Therefore, we can consider lifelong learning simply as a special case of online or incremental learning, in case of deep neural networks.
There are multiple ways to perform such incremental learning BID12 BID17 .
The simplest way is to incrementally fine-tune the network to new tasks by continuing to train the network with new training data.
However, such simple retraining of the network can degenerate the performance for both the new tasks and the old ones.
If the new task is largely different from the older ones, such as in the case where previous tasks are classifying images of animals and the new task is to classify images of cars, then the features learned on the previous tasks may not be useful for the new one.
At the same time, the retrained representations for the new task could adversely affect the old tasks, as they may have drifted from their original meanings and are no longer optimal for them.
For example, the feature describing stripe pattern from zebra, may changes its meaning for the later classification task for classes such as striped t-shirt or fence, which can fit to the feature and drastically change its meaning.Then how can we ensure that the knowledge sharing through the network is beneficial for all tasks, in the online/incremental learning of a deep neural network?
Recent work suggests to either use a regularizer that prevents the parameters from drastic changes in their values yet still enables to find a good solution for the new task BID4 , or block any changes to the old task BID4 retrains the entire network learned on previous tasks while regularizing it to prevent large deviation from the original model.
Units and weights colored in red denote the ones that are retrained, and black ones are ones that remain fixed.
(b) Non-retraining models such as Progressive Network BID12 expands the network for the new task t, while withholding modification of network weights for previous tasks.
(c) Our DEN selectively retrains the old network, expanding its capacity when necessary, and thus dynamically deciding its optimal capacity as it trains on.
parameters BID12 .
Our strategy is different from both approaches, since we retrain the network at each task t such that each new task utilizes and changes only the relevant part of the previous trained network, while still allowing to expand the network capacity when necessary.
In this way, each task t will use a different subnetwork from the previous tasks, while still sharing a considerable part of the subnetwork with them.
FIG0 illustrates our model in comparison with existing deep lifelong learning methods.There are a number of challenges that need to be tackled for such incremental deep learning setting with selective parameter sharing and dynamic layer expansion.
1) Achieving scalability and efficiency in training: If the network grows in capacity, training cost per task will increasingly grow as well, since the later tasks will establish connections to a much larger network.
Thus, we need a way to keep the computational overhead of retraining to be low.2) Deciding when to expand the network, and how many neurons to add: The network might not need to expand its size, if the old network sufficiently explains the new task.
On the other hand, it might need to add in many neurons if the task is very different from the existing ones.
Hence, the model needs to dynamically add in only the necessary number of neurons.3) Preventing semantic drift, or catastrophic forgetting, where the network drifts away from the initial configuration as it trains on, and thus shows degenerate performance for earlier examples/tasks.
As our method retrains the network, even partially, to fit to later learned tasks, and add in new neurons which might also negatively affect the prior tasks by establishing connections to old subnetwork, we need a mechanism to prevent potential semantic drift.To overcome such challenges, we propose a novel deep network model along with an efficient and effective incremental learning algorithm, which we name as Dynamically Expandable Networks (DEN).
In a lifelong learning scenario, DEN maximally utilizes the network learned on all previous tasks to efficiently learn to predict for the new task, while dynamically increasing the network capacity by adding in or splitting/duplicating neurons when necessary.
Our method is applicable to any generic deep networks, including convolutional networks.We validate our incremental deep neural network for lifelong learning on multiple public datasets, on which it achieves similar or better performance than the model that trains a separate network for each task, while using only 11.9%p − 60.3%p of its parameters.
Further, fine-tuning of the learned network on all tasks obtains even better performance, outperforming the batch model by as much as 0.05%p − 4.8%p.
Thus, our model can be also used for structure estimation to obtain optimal performance over network capacity even when batch training is possible, which is a more general setup.
We proposed a novel deep neural network for lifelong learning, Dynamically Expandable Network (DEN).
DEN performs partial retraining of the network trained on old tasks by exploiting task relatedness, while increasing its capacity when necessary to account for new knowledge required to account for new tasks, to find the optimal capacity for itself, while also effectively preventing semantic drift.
We implement both feedforward and convolutional neural network version of our DEN, and validate them on multiple classification datasets under lifelong learning scenarios, on which they significantly outperform the existing lifelong learning methods, achieving almost the same performance as the network trained in batch while using as little as 11.9%p − 60.3%p of its capacity.
Further fine-tuning of the models on all tasks results in obtaining models that outperform the batch models, which shows that DEN is useful for network structure estimation as well.

assistant: text: We propose a novel deep network architecture that can dynamically decide its network capacity as it trains on a lifelong learning scenario.