Title: Softmax Q-Distribution Estimation for Structured Prediction: A Theoretical Interpretation for RAML,
Abstract: Reward augmented maximum likelihood (RAML), a simple and effective learning
framework to directly optimize towards the reward function in structured
prediction tasks, has led to a number of impressive empirical successes. RAML
incorporates task-specific reward by performing maximum-likelihood updates on
candidate outputs sampled according to an exponentiated payoff distribution,
which gives higher probabilities to candidates that are close to the reference
output. While RAML is notable for its simplicity, efficiency, and its
impressive empirical successes, the theoretical properties of RAML, especially
the behavior of the exponentiated payoff distribution, has not been examined
thoroughly. In this work, we introduce softmax Q-distribution estimation, a
novel theoretical interpretation of RAML, which reveals the relation between
RAML and Bayesian decision theory. The softmax Q-distribution can be regarded
as a smooth approximation of the Bayes decision boundary, and the Bayes
decision rule is achieved by decoding with this Q-distribution. We further show
that RAML is equivalent to approximately estimating the softmax Q-distribution,
with the temperature $\tau$ controlling approximation error. We perform two
experiments, one on synthetic data of multi-class classification and one on
real data of image captioning, to demonstrate the relationship between RAML and
the proposed softmax Q-distribution estimation method, verifying our
theoretical analysis. Additional experiments on three structured prediction
tasks with rewards defined on sequential (named entity recognition), tree-based
(dependency parsing) and irregular (machine translation) structures show
notable improvements over maximum likelihood baselines.