The Predictron: End-To-End Learning and Planning2017 Jan 04
One of the key challenges of artificial intelligence is to learn models that are effective in the context of planning. In this document we introduce the predictron architecture. The predictron consists of a fully abstract model, represented by a Markov reward process, that can be rolled forward multiple “imagined” planning steps. Each forward pass of the predictron accumulates internal rewards and values over multiple planning depths. The predictron is trained end-to-end so as to make these accumulated values accurately approximate the true value function. We applied the predictron to procedurally generated random mazes and a simulator for the game of pool. The predictron yielded significantly more accurate predictions than conventional deep neural network architectures.
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The predictron is a single differentiable architecture that rolls forward an internal model to estimate values. This internal model may be given both the structure and the semantics of traditional reinforcement learning models. But unlike most approaches to model-based reinforcement learning, the model is fully abstract: it need not correspond to the real environment in any human understandable fashion, so long as its rolled-forward “plans” accurately predict outcomes in the true environment. The predictron may be viewed as a novel network architecture that incorporates several separable ideas. First, the predictron outputs a value by accumulating rewards over a series of internal planning steps. Second, each forward pass of the predictron outputs values at multiple planning depths. Third, these values may be combined together, also within a single forward pass, to output an overall ensemble value. Finally, the different values output by the predictron may be encouraged to be self-consistent with each other, to provide an additional signal during learning. Our experiments demonstrate that these differences result in more accurate predictions of value, in reinforcement learning environments, than more conventional network architectures. We have focused on value prediction tasks in uncontrolled environments. However, these ideas may transfer to the control setting, for example by using the predictron as a Q-network (Mnih et al., 2015). Even more intriguing is the possibility of learning an internal MDP with abstract internal actions, rather than the MRP model considered in this paper. We aim to explore these ideas in future work.