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Title: Neural Network Bandit Learning by Last Layer Marginalization

Paper content:
We propose a new method for training neural networks online in a bandit setting.
Similar to prior work, we model the uncertainty only in the last layer of the network, treating the rest of the network as a feature extractor.
This allows us to successfully balance between exploration and exploitation due to the efficient, closed-form uncertainty estimates available for linear models.
To train the rest of the network, we take advantage of the posterior we have over the last layer, optimizing over all values in the last layer distribution weighted by probability.
We derive a closed form, differential approximation to this objective and show empirically over various models and datasets that training the rest of the network in this fashion leads to both better online and offline performance when compared to other methods.
Applying machine learning models to real world applications almost always involves deploying systems in dynamic, non-stationary environments.
This dilemma requires models to be constantly re-updated with new data in order to maintain a similar model performance across time.
Of course, doing this usually requires the new data to be relabeled, which can be expensive or in some cases, impossible.
In many situations, this new labeled data can be cheaply acquired by utilizing feedback from the user, where the feedback/reward indicates the quality of the action taken by the model for the given input.
Since the inputs are assumed independent, this task can be framed in the contextual bandit setting.
Learning in this setting requires a balance between exploring uncertain actions (where we risk performing sub optimal actions) and exploiting actions the model is confident will lead to high reward (where we risk missing out on discovering better actions).Methods
based on Thompson sampling (TS) or Upper Confidence Bounds (UCB) provide theoretically BID1 BID0 and empirically established ways BID12 BID3 for balancing exploration/exploitation in this setting. Unfortunately
, both methods require estimation of model uncertainty. While this can
be done easily for most linear models, it is a difficult and open problem for large neural network models underlying many modern machine learning systems. An empirical study
by BID17 shows that having good uncertainty estimates is vital for neural networks learning in a bandit setting. Closed formed uncertainty
estimations (and online update formulas) are available for many linear models. Since the last layer of many
neural networks are usually (generalized) linear models, a straightforward way for learning neural networks in a bandit setting is to estimate the uncertainty (as a distribution over weights) on the last layer only, holding the previous layers fixed as feature functions which provide inputs to the linear model. This method (and variants thereof
) has been proposed in bandit settings BID17 BID13 as well as other related settings (Snoek et al., 2015; BID16 BID11 and has been shown to work surprisingly well considering its simplicity. This style of methods, which we
refer to as Bayesian last layer or BLL methods, also has the advantage of being both relatively model-agnostic and scalable to large models. Of course, BLL methods come with
the tacit assumption that the feature functions defined by the rest of the network output good (linearly separable) representations of our inputs. This means that, unless the input
data distribution is relatively static, the rest of the network will need to be updated in regular intervals to maintain low regret.In order to maintain low regret, the retraining objective must: 1) allow new data to be incorporated
quickly into the learned model, and 2) prevent previously learned information
from being quickly forgotten. Previous papers retrain BLL methods simply
by sampling minibatches from the entire pool of previously seen data and maximizing log-likelihood over these minibatches, which fails to meet the first criteria above.In this paper we present a new retraining objective for BLL methods meeting both requirements. We avoid retraining the last layer with the
entire network (throwing out the uncertainty information we learned about the last layer) or retraining with the last layer fixed (fixing the last layer to the mean of its distribution). Instead, we utilize the uncertainty information
gathered about the last layer, and optimize the expected log-likelihood of both new and old data, marginalizing 1 over the entire distribution we have on the last layer. This gives a more robust model that performs relatively
well over all likely values of the last layer. While this objective cannot be exactly computed, we derive
a closed form, differentiable, approximation. We show that this approximation meets both criteria above,
with a likelihood term to maximize that depends only on the new data (meeting the first point), and a quadratic regularization term that is computed only with previously seen data (meeting the second point). We show empirically that this method improves regret on the
most difficult bandit tasks studied in BID17 . We additionally test the method on a large state-of-the-art
recurrent model, creating a bandit task out of a paraphrasing dataset. Finally, we test the method on convolutional models, constructing
a bandit task from a benchmark image classification dataset. We show that our method is fast to adapt to new data without quickly
forgetting previous information.
As previously done in BID17 , we use a two layer MLP as the underlying model, using the same configuration across all methods.
For Marginalize and Sample New, we perform the retraining after 1000 rounds.
For Sample All we update after 100 rounds just like BID17 .
In Table 1 we report the average cumulative regret as well as the cumulative regret relative to a policy that selects arms uniformly at random.
We report the results for both Thompson Sampling (TS) and for UCB policies.
Results are similar for either UCB and TS which shows that policies does not influence performance of the training mechanisms.On most of the tasks both Marginalize (our method) and Sample New outperforms Sample All (method used in BID17 ) in terms of cumulative regret.
Both Marginalize and Sample New techniques are very similar in performance for the three datasets.
All the three datasets used in this experiment are low dimensional, static, and relatively easy to learn, hence there is not much history to retain for Sample New technique.
In the next section we will present results on larger datasets and also evaluate where we will show that our method performs better than Sample New.
In TAB2 we show that our method Marginalize outperforms both Sample All and Sample New techniques for both multiclass and pool based tasks.
Sample All and Sample New have comparable cumulative regret.
Sample New has worse offline accuracy on Quora dataset (because it forgets old information), while it has better offline accuracy on MSR (because it is able to adapt quicker).
For Batch train, both multiclass and pool based tasks are same-a binary classification problem.
Batch train performs only slightly better than our method in terms of offline accuracy, where Batch train gets full feedback, while our method only gets partial (bandit) feedback.
FIG1 further shows that when the data distribution changes (switching form Quora to MSR in the pool based task) Marginalize and Sample New are able to adapt much faster than Sample All.
Overall Marginalize achieved a lower regret as well as higher offline accuracy for both the bandit settings.
In Table 3 we present results for the image classification bandit task, using average cumulative regret and offline accuracy as evaluation metrics.
Again, Sample New performs better than Sample All for cumulative regret but under-performs in the offline setting.
As expected, our method performs well for both cumulative regret and offline setting.
For the multiclass task, our method performs significantly lower than batch train.
This is not too surprising, for two reasons:
i) Training a CNN architecture takes many more epochs over the data to converge (∼ 20 in our case) which is not achieved in a bandit setting;
ii) CIFAR-10 has 10 classes, each defining an arm and in our setting; the bandit algorithms only gets feedback for one class in each round, compared to the full feedback received in batch train.
Effectively, the number of labels per class in cut by a factor of 10.
This is not as much an issue in the pool task, where we can see the results between batch train and the bandit algorithms are comparable.
In this paper we proposed a new method for training neural networks in a bandit setting.
We tackle the problem of exploration-exploitation by estimating uncertainty only in the last layer, allowing the method to scale to large state-of-the-art models.
We take advantage of having a posterior over the last layer weights by optimizing the rest of the network over all values of the last layer.
We show that method outperforms other methods across a diverse set of underlying models, especially in online tasks where the distribution shifts rapidly.
We leave it as future work to investigate more sophisticated methods for determining when to retrain the network, how to set the weight (β) of the regularization term in a more automatic way, and its possible connections to methods used for continual learning.

assistant: text: This paper proposes a new method for neural network learning in online bandit settings by marginalizing over the last layer