Yunhao (Robin) Tang I am a research scientist at DeepMind London. Previously, I obtained my Ph.D. from Columbia University, where I was very fortunate to be advised by Prof. Shipra Agrawal. Before that, I received my M.S. in Financial Engineering at Columbia University and my B.S. in Physics at Fudan University. In Summer 2021, I am an intern at the Deep RL team in DeepMind Paris remotely from New York. In Fall 2019, I had a wonderful time working as an intern at DeepMind Paris hosted by Remi Munos. I worked on topics related to large-scale distributed reinforcement learning agents, mostly off-policy, some model-free and some model-based.
 News [9/2021] One paper accepted at NeurIPS 2021. Please check it out here! [8/2021] I defended my PhD thesis. Many thanks to my advisor, my committee members, my mentors and collaborators. [2/2021] Two papers accepted at ICML 2021. Many thanks to my collaborators! [2/2021] I have the great pleasure to present our previous work on RL for integer programming at the IPAM workshop held by UCLA. Both the video and slides are available here. [1/2021] One paper accepted at AISTATS 2021. Please check it out here! [9/2020] One paper accepted at NeurIPS 2020. Please check it out here! [6/2020] Four papers accepted at ICML 2020. I am grateful to having all the wonderful coauthors! [5/2020] Our paper wins the most popular poster and honorable mention of the best paper at MIP 2020. The video presentation is here.
 Research I have a broad interest in machine learning in general, in particular in reinforcement learning. For a selected subset of publications, see highlighted papers below.
 Unifying Gradient Estimators for Meta-Reinforcement Learning via Off-Policy Evaluation Yunhao Tang*, Tadashi Kozuno*, Mark Rowland, Remi Munos, Michal Valko Neural Information Processing Systems (NeurIPS), Virtual, 2021 arXiv How to estimate high-order derivatives of value functions? We propose a unifying framework with off-policy evaluation. Direct differentiations of off-policy estimates produce estimates to high-order derivatives of value functions, and instantiate many prior methods as special cases. Marginalized Operators for Off-policy Reinforcement Learning Yunhao Tang, Mark Rowland, Remi Munos, Michal Valko Workshop on Reinforcement Learning Theory, International Conference on Machine Learning, 2021 paper Can we combine marginalized estimation with contractive operators for off-policy learning? Taylor Expansion of Discount Factors Yunhao Tang, Mark Rowland, Remi Munos, Michal Valko International Conference on Machine Learning (ICML), Virtual, 2021 arXiv In deep RL practices, we estimate discounted value functions with a small discount factors, yet at evaluation time we care about the undiscounted objective with a large effective discount factor. We make clear the connections between value functions of different discount factors, and partially justify some ubiquitous deep RL heuristics. Revisiting Peng's Q($\lambda$) for Modern Reinforcement Learning Tadashi Kozuno*, Yunhao Tang*, Mark Rowland, Remi Munos, Steven Kapturowski, Will Dabney, Michal Valko, David Abel International Conference on Machine Learning (ICML), Virtual, 2021 ArXiv Code Uncorrected multi-step updates such as n-step Q-learning are ubiquitous in modern deep RL practices. We revisit Peng's Q($\lambda$), a classic uncorrected multi-step variant. Our analysis sheds light on why uncorrected updates should work in practice. The empirical result also suggests significant gains on benchmark tasks. Hindsight Expectation Maximization for Goal-conditioned Reinforcement Learning Yunhao Tang, Alp Kucukelbir International Conference on Artificial Intelligence and Statistics (AISTATS), Virtual, 2021 paper / arXiv We propose Hindsight Expectation Maximization (hEM), an EM algorithm for goal-conditioned RL problem which combines supervised learning through the M-step and hindsight goal sampling through the E-step. We also make an intimate connection between hindsight replay and importance sampling for rare event simulations. Self-Imitation Learning via Generalized Lower Bound Q-learning Yunhao Tang Neural Information Processing Systems (NeurIPS), Vancouver, Canada, 2020 paper / arXiv Why is self-imitation learning efficient? We shed light on its connections to n-step Q-learning and show that part of its gains might be attributed to trade-offs in RL operators. We also propose a n-step extension of self-imitation learning which incorporates the strengths of both n-step updates and lower-bound learning. Monte-Carlo Tree Search as Regularized Policy Optimization Jean-Bastien Grill*, Florent Altche*, Yunhao Tang*, Thomas Hubert, Michal Valko, Ioannis Antonoglou, Remi Munos International Conference on Machine Learning (ICML), Vienna, Austria, 2020 paper / arXiv / video We establish an interpretation of MCTS as policy optimization. This interpretation leads to algorithmic variants which naturally improve over MCTS-based baselines such as AlphaZero and MuZero. Taylor Expansion Policy Optimization Yunhao Tang, Michal Valko, Remi Munos International Conference on Machine Learning (ICML), Vienna, Austria, 2020 paper / arXiv / video / media 1 / media 2 We estabilish the intimate connections between trust region policy search and off-policy evaluation. The new algorithm TayPO generalizes policy optimization objectives to high-order extentions which leads to gains on large-scale distributed agents. Reinforcement Learning for Integer Programming: Learning to Cut Yunhao Tang, Shipra Agrawal, Yuri Faenza International Conference on Machine Learning (ICML), Vienna, Austria, 2020 paper / arXiv / video We formulate cutting plane algorithms as a sequential decision making problem for generic integer pgroamming. The cutting plane agent learned via RL improves over human-designed heuristics and benfits downstream applications such as branch-and-cut. Online Hyper-parameter Tuning in Off-policy Learning via Evolutionary Strategies Yunhao Tang, Krzysztof Choromanski Workshop on Deep Reinforcement Learning, Neural Information Processing Systems (NeurIPS), Vancouver, Canada, 2020 paper / arXiv We carry out online hyper-parameter adaptation of off-policy learning algorithms with Evolutionary Strategies. The overall algorithm outperforms baselines with static hyper-parameters. Guiding Evolutionary Strategies with Off-policy Actor-critic Yunhao Tang International Conference on Autonomous Agents and Multiagent Systems (AAMAS), London, United Kingdom, 2021 paper We use off-policy policy gradient to guide the random search of evolutionary algorithms. This provides more informative updates which outperform baselines. Learning to Score Behaviors for Guided Policy Optimization Aldo Pacchiano*, Jack Parker-Holder*, Yunhao Tang*, Anna Choromanska, Krzysztof Choromanski, Michael Jordan International Conference on Machine Learning (ICML), Vienna, Austria, 2020 paper / arXiv We propose to semantically score trajectories of RL agents with Wasserstein distance. The new algorithmic variant improves upon previous alternative distance metrics. Variance Reduction for Evolutionary Strategies via Structured Control Variates Yunhao Tang, Krzysztof Choromanski, Alp Kucukelbir International Conference on Artificial Intelligence and Statistics (AISTATS), Palermo, Italy, 2020 paper / arXiv We propose a RL-specific control variate for variance reduction of Evolutionary Strategies. This method outperforms previous general purpose variance reduction techniques. Discrete Action On-Policy Learning with Action-Value Critic Yuguang Yue, Yunhao Tang, Mingzhang Yin and Mingyuan Zhou International Conference on Artificial Intelligence and Statistics (AISTATS), Palermo, Italy, 2020 paper / arXiv We propose a novel variance reduction technique based on the Augment-Reinforce-Swap-Merge (ARSM) framework for on-policy optimization in RL. Practical Nonisotropic Monte Carlo Sampling in High Dimensions via Determinantal Point Processes Krzysztof Choromanski*, Aldo Pacchiano*, Jack Parker-Holder*, Yunhao Tang* International Conference on Artificial Intelligence and Statistics (AISTATS), Palermo, Italy, 2020 paper / arXiv We propose a sampling scheme which incorporates determinantal point process into Evolutionary Strategies for variance reduction. Discretizing Continuous Action Space for On-Policy Optimization Yunhao Tang, Shipra Agrawal Association for the Advancement of Artificial Intelligence (AAAI), New York, NY, USA, 2020 paper / arXiv / code The simple idea of discretizing the continuous control action space into a discrete space works decently. ES-MAML: Simple Hessian-Free Meta Learning Xingyou Song, Wenbo Gao, Yuxiang Yang, Krzysztof Choromanski, Aldo Pacchiano, Yunhao Tang International Conference on Learning Representations (ICLR), Addis Ababa, Ethiopia, 2020 arXiv Evolutionary Strategies can be applied to meta-learning and works decently compared to popular gradient-based alternatives. The new formulation also easily scales with computational resources. Augment-Reinforce-Merge Policy Gradient for Binary Stochastic Policy Yunhao Tang, Mingzhang Yin, Mingyuan Zhou arXiv 2019 arXiv We show that when combined with the Augment-Reinforce-Merge (ARM) technique for variance reduction, the performance of policy gradient algorithms with binary action space could be significantly improved. Provably Robust Blackbox Optimization for Reinforcement Learning Krzysztof Choromanski*, Aldo Pacchiano*, Jack Parker-Holder*, Yunhao Tang, Deepali Jain, Yuxiang Yang, Atil Iscen, Jasmine Hsu, Vikas Sindhwani Conference on Robot Learninng (CORL), spotlight, Okasa, Japan, 2019 paper / arXiv We propose to combine robust regression with Evolutionary Strategies. This new formulation naturally entails sample reuse and improves the robustness of the overall algorithm. From Complexity to Simplicity: Adaptive ES-Active Subspaces for Blackbox Optimization Krzysztof Choromanski*, Aldo Pacchiano*, Jack Parker-Holder*, Yunhao Tang*, Vikas Sindhwani Neural Information Processing Systems (NeurIPS), Vancouver, Canada, 2019 paper / arXiv We propose to constrain the random search employed by Evolutionary Strategies into adaptive subspaces, which entails improved sample efficiency. KAMA-NNs: Low-Dimensional Rotation based Neural Networks Krzysztof Choromanski*, Aldo Pacchiano*, Jeffrey Pennington*, Yunhao Tang* International Conference on Artificial Intelligence and Statistics (AISTATS), Okinawa, Japan, 2019 paper / supplementary We design a novel neural network architecture based on Kac's random walk. This new architecture greatly reduces the number of parameters yet retains competitive performance on RL tasks compared to fully-connected architecture. Orthogonal Estimation of Wasserstein Distances Mark Rowland*, Jiri Hron*, Yunhao Tang*, Krzysztof Choromanski, Tamas Sarlos, Adrian Weller International Conference on Artificial Intelligence and Statistics (AISTATS), Okinawa, Japan, 2019 paper / supplementary / arXiv We propose an orthogonal sampling method for computing sliced Wasserstein distance. This new technique entails variance reduction and imporves training of downstream tasks. Exploration by Distributional Reinforcement Learning Yunhao Tang, Shipra Agrawal International Joint Conference on Artificial Intelligence (IJCAI), Stockholm, Sweden, 2018 arXiv / code We carry out approximate Thomson sampling with randomized DQN for efficient exploration in RL.
 Additional Preprints and Peer-reviewed Workshop Papers
 Reinforcement Learning with Chromatic Networks Xingyou Song, Krzysztof Choromanski, Jack Parker-Holder, Yunhao Tang, Wenbo Gao, Aldo Pacchiano, Tamas Sarlos, Deepali Jain, Yuxiang Yang Workshop on Neural Architecture Search, International Conference on Learning Representations (ICLR), Addis Ababa, Ethiopia, 2020 arXiv We show that to achieve decent performance on continuous control RL benchmark tasks only requires a small number of parameters. Combining Model-based and Model-free Reinforcement Learning through Evolution Yunhao Tang Deep Reinforcement Learning Workshop, Neural Information Processing Systems (NeurIPS), Vancouver, Canada, 2018 paper We show that combinng Evolutionary Strategies with model-based RL methods has the potential of brining the best of both worlds. Variational Auto-encoding Contexts for Control Yunhao Tang, Xiya Cao Inference to Control Workshop, Neural Information Processing Systems (NeurIPS), Vancouver, Canada, 2018 paper We show that incorporating generative modeling techniques into the RL loss improves the performance for certain partially-observable tasks. Boosting Trust Region Policy Optimization with Normalizing Flows Policy Yunhao Tang, Shipra Agrawal Invertible Neural Nets and Normalizing Flows Workshop, International Conference on Machine Learning (ICML), Long Beach, California, USA, 2019 Deep Reinforcement Learning Workshop, Neural Information Processing Systems (NeurIPS), Montreal, Canada, 2018 arXiv We show that the performance of trust region policy search could be signicantly improved via expressive policy classes, e.g. normalizing flows. Variational Deep Q Network Yunhao Tang, Alp Kucukelbir Bayesian Deep Learning Workshop, Neural Information Processing Systems (NeurIPS), Long Beach, California, USA, 2017 arXiv / code We show that introducing structured randomness in DQN agents entails more efficient exploration. Implicit Policy for Reinforcement Learning Yunhao Tang, Shipra Agrawal arXiv 2018 arXiv We demonstrate the utility of implicit distributions as RL policy.
 Teaching assistant
 IEOR 4525, Machine Learning for OR and FE, Spring 2020 IEOR 8100, Reinforcement Learning, Spring 2019 and Spring 2018 IEOR 4525, Machine Learning for OR and FE, Fall 2018 COMS 6998, Probabilistic Programming, Fall 2018 IEOR 6711, Stochastic Modeling I, Fall 2017
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