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Title: Policy Gradient For Multidimensional Action Spaces: Action Sampling and Entropy Bonus

Paper content:
In recent years deep reinforcement learning has been shown to be adept at solving sequential decision processes with high-dimensional state spaces such as in the Atari games.
Many reinforcement learning problems, however, involve high-dimensional discrete action spaces as well as high-dimensional state spaces.
In this paper, we develop a novel policy gradient methodology for the case of large multidimensional discrete action spaces.
We propose two approaches for creating parameterized policies: LSTM parameterization and a Modified MDP (MMDP) giving rise to Feed-Forward Network (FFN) parameterization.
Both of these approaches provide expressive models to which backpropagation can be applied for training.
We then consider entropy bonus, which is typically added to the reward function to enhance exploration.
In the case of high-dimensional action spaces, calculating the entropy and the gradient of the entropy requires enumerating all the actions in the action space and running forward and backpropagation for each action, which may be computationally infeasible.
We develop several novel unbiased estimators for the entropy bonus and its gradient.
Finally, we test our algorithms on two environments: a multi-hunter multi-rabbit grid game and a multi-agent multi-arm bandit problem.
In recent years deep reinforcement learning has been shown to be adept at solving sequential decision processes with high-dimensional state spaces such as in the Go game ) and Atari games BID6 , BID7 , BID9 , BID11 , BID12 , BID22 , BID1 ).
In all of these success stories, the size of the action space was relatively small.
Many reinforcement learning problems, however, involve high-dimensional action spaces as well as high-dimensional state spaces.
Examples include StarCraft BID21 , BID4 ), where there are many agents each of which can take a finite number of actions; and coordinating self-driving cars at an intersection, where each car can take a finite set of actions BID17 ).In
this paper, we develop a novel policy gradient methodology for the case of large multidimensional action spaces. There
are two major challenges in developing such a methodology:• For large multidimensional action spaces, how can we design expressive and differentiable parameterized policies which can be efficiently sampled?• In policy
gradient, in order to encourage sufficient exploration, an entropy bonus term is typically added to the objective function. However, in
the case of high-dimensional action spaces, calculating the entropy and its gradient requires enumerating all the actions in the action space and running forward and backpropagation for each action, which may be computationally infeasible. How can we
efficiently approximate the entropy and its gradient while maintaining desirable exploration?In this paper
, we first propose two approaches for parameterizing the policy: a LSTM model and a Modified MDP (MMDP) giving rise to Feed-Forward Network (FFN) model. For both of
these parameterizations, actions can be efficiently sampled from the policy distribution, and backpropagation can be employed for training. We then develop
several novel unbiased estimators for the entropy bonus and its gradient. These estimators
can be combined with stochastic gradient descent giving a new a class of policy gradient algorithms with desirable exploration. Finally, we test
our algorithms on two environments: a multi-agent multi-arm bandit problem and a multi-agent hunter-rabbit grid game.
In this paper, we developed a novel policy gradient methodology for the case of large multidimensional discrete action spaces.
We proposed two approaches for creating parameterized policies: LSTM parameterization and a Modified MDP (MMDP) giving rise to Feed-Forward Network (FFN) parameterization.
Both of these approaches provide expressive models to which backpropagation can be applied for training.
We then developed several novel unbiased estimators for entropy bonus and its gradient.
We did experimental work for two environments with large multidimensional action space.
For these environments, we found that both the LSTM and MMDP approach could successfully solve large multidimensional action space problems, with the LSTM approach generally performing better.
We also found that the smoothed estimates of the entropy and the unbiased gradient estimate of the entropy gradient can help reduce computational cost while not sacrificing significant loss in performance.

assistant: text: policy parameterizations and unbiased policy entropy estimators for MDP with large multidimensional discrete action space