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Title: Buy 4 REINFORCE Samples, Get a Baseline for Free!

Paper content:
REINFORCE can be used to train models in structured prediction settings to directly optimize the test-time objective.
However, the common case of sampling one prediction per datapoint (input) is data-inefficient.
We show that by drawing multiple samples (predictions) per datapoint, we can learn with significantly less data, as we freely obtain a REINFORCE baseline to reduce variance.
Additionally we derive a REINFORCE estimator with baseline, based on sampling without replacement.
Combined with a recent technique to sample sequences without replacement using Stochastic Beam Search, this improves the training procedure for a sequence model that predicts the solution to the Travelling Salesman Problem.
REINFORCE (Williams, 1992 ) is a well known policy optimization algorithm that learns directly from experience.
Variants of it have been used to train models for a wide range of structured prediction tasks, such as Neural Machine Translation BID12 BID0 , Image Captioning (Vinyals et al., 2015b) and predicting solutions (tours) for the Travelling Salesman Problem (TSP) BID1 BID6 .
As opposed to maximum likelihood (supervised) learning, the appeal of using REINFORCE for structured prediction is that it directly optimizes the test-time performance.When using REINFORCE, often for each datapoint (e.g. a sentence, image or TSP instance) only a single sample/prediction (e.g. a translation, caption or tour) is used to construct a gradient estimate.
From a classic Reinforcement Learning (RL) point of view, this makes sense, as we may not be able to evaluate multiple sampled actions for a state (datapoint).
However, from a data point of view, this is inefficient if we can actually evaluate multiple samples, such as in a structured prediction setting.
Reinforcement Learning with multiple samples/predictions for a single datapoint has been used before (e.g. BID14 ; ), but we use the samples as counterfactual information by constructing a (local, for a single datapoint) REINFORCE baseline.
A similar idea was applied for variational inference by BID10 .Many
structured prediction tasks can be formulated in terms of sequence modelling, which is the focus of this paper. In most
sequence modelling tasks, the objective is a deterministic function of the predicted sequence. As a result
, duplicate sampled sequences are uninformative and therefore do not improve the quality of the gradient estimate. To solve this
problem, we propose to use sampling without replacement to construct a better gradient estimate. This is inspired
by recent work by BID7 , who introduce Stochastic Beam Search as a method to sample sequences without replacement, and use this to construct a (normalized) importance-weighted estimator for (sentence level) BLEU score. We extend this idea
to estimate policy gradients using REINFORCE, and we show how to use the same set of samples (without replacement) to construct a baseline. This way we can leverage
sampling without replacement to improve training of sequence models.In our experiment, we consider the TSP and show that using REINFORCE with multiple samples is beneficial compared to single sample REINFORCE, both computationally and in terms of data-efficiency. Additionally, for a sample
size of 4 − 8 samples per datapoint, sampling without replacement results in slightly faster learning.
In this paper, we have derived REINFORCE estimators based on drawing multiple samples, with and without replacement, and evaluated the effectiveness of the proposed estimators in a structured prediction setting: the prediction of tours for the TSP.
The derived estimators yield results comparable to recent results using REINFORCE with a strong greedy rollout baseline, at greater data-efficiency and computational efficiency.These estimators are especially well suited for structured prediction settings, where the domain is too large to compute exact gradients, but we are able to take multiple samples for the same datapoint, and the objective is a deterministic function of the sampled prediction.
We hope the proposed estimators have potential to be used to improve training efficiency in more structured prediction settings, for example in the context of Neural Machine Translation or Image Captioning, where depending on the entropy of the model, sampling without replacement may yield a beneficial improvement.

assistant: text: We show that by drawing multiple samples (predictions) per input (datapoint), we can learn with less data as we freely obtain a REINFORCE baseline.