Methods, systems and apparatus, including computer programs encoded on computer storage media, for training a value neural network that is configured to receive an observation characterizing a state of an environment being interacted with by an agent and to process the observation in accordance with parameters of the value neural network to generate a value score. One of the systems performs operations that include training a supervised learning policy neural network; initializing initial values of parameters of a reinforcement learning policy neural network having a same architecture as the supervised learning policy network to the trained values of the parameters of the supervised learning policy neural network; training the reinforcement learning policy neural network on second training data; and training the value neural network to generate a value score for the state of the environment that represents a predicted long-term reward resulting from the environment being in the state.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A system comprising one or more computers and one or more storage devices storing instructions that when executed by the one or more computers cause the one or more computers to perform operations comprising: maintaining data representing a state tree of an environment being interacted with by an agent by performing actions selected from a set of possible actions, wherein the state tree comprises a plurality of nodes and a plurality of edges, wherein each node represents a respective state of the environment that has been encountered by the agent during interaction with the environment or with a simulated version of the environment, and wherein each edge connects a respective first node to a respective second node and represents an action that (i) was performed by the agent in response to an observation characterizing a first state represented by the respective first node and (ii) resulted in the environment or the simulated version of the environment transitioning into a second state represented by the respective second node; maintaining edge data, the edge data comprising, for each of the plurality of edges: (i) a respective action score for the action represented by the edge, (ii) a respective visit count that represents a number of times that the action represented by the edge has been performed by the agent in response to observations characterizing the respective first state represented by the respective first node for the edge, and (iii) a respective prior probability; receiving a root observation characterizing a root state of the environment that is represented by a root node of the plurality of nodes; selecting actions to be performed by the agent to interact with the environment or the simulated version of the environment by traversing the state tree from the root node according to the edge data until the environment or the simulated version of the environment reaches a leaf state represented by a leaf node in the state tree; and evaluating the leaf node by processing a leaf observation characterizing the leaf node using a value neural network that has been trained to process the leaf observation to generate a value score for the leaf state that represents a predicted long-term reward received as a result of the environment being in the leaf state. 2. The system of claim 1, wherein the environment is a real-world environment, and wherein the actions in the set of actions are possible control inputs to control the interaction of the agent with the environment. 3. The system of claim 2, wherein the environment is a real-world environment, wherein the agent is a control system for an autonomous or semi-autonomous vehicle navigating through the real-world environment, and wherein the actions in the set of actions are possible control inputs to control the autonomous or semi-autonomous vehicle. 4. The system of claim 2, wherein the predicted long-term reward received by the agent reflects a predicted degree to which objectives for the navigation of the vehicle through the real-world environment will be satisfied as a result of the environment being in the leaf state. 5. The system of claim 1, wherein the environment is a patient diagnosis environment, wherein each state is a respective patient state of a patient, wherein the agent is a computer system for suggesting treatment for the patient, and wherein the actions in the set of actions are possible medical treatments. 6. The system of claim 1, wherein the environment is a protein folding environment, wherein each state is a respective state of a protein chain, wherein the agent is a computer system for determining how to fold the protein chain, and wherein the actions are possible folding actions for folding the protein chain. 7. The system of claim 1, wherein the environment is a virtual environment in which a user competes against a computerized agent to accomplish a goal, wherein the agent is the computerized agent, and wherein the actions in the set of actions are possible actions that can be performed by the computerized agent. 8. The system of claim 1, the operations further comprising: evaluating the leaf node by performing a rollout until the environment or the simulated version of the environment reaches a terminal state, comprising: for each state encountered by the agent during the rollout: receiving rollout data characterizing the state; processing the rollout data using a fast rollout policy neural network that has been trained to receive the rollout data and to process the rollout data to generate a respective rollout action probability for each action in the set of possible actions; and selecting an action as an action to be performed by the agent in response to the rollout data characterizing the state using the respective rollout action probabilities; and generating a rollout long-term reward based on the terminal state. 9. The system of claim 8, the operations further comprising: generating a leaf evaluation score by combining the rollout long-term reward and the predicted long-term reward; and updating the edge data for edges traversed during the traversal based on the leaf evaluation score. 10. The system of claim 9, wherein updating the edge data comprises, for each edge that was traversed: incrementing the visit count for the edge; and updating the action score for the edge using the leaf evaluation such that the action score is an average of leaf evaluation scores for each time the edge has been traversed. 11. The system of claim 10, the operations further comprising: adding a respective new edge to the state tree for each action that is a valid action to be performed by the agent in response to the leaf observation. 12. The system of claim 11, the operations further comprising: generating edge data for each new edge, comprising generating a posterior probability for the new edge by processing the leaf observation using a policy neural network, wherein the policy neural network is configured to process the leaf observation to generate a respective action probability for each action in the set of possible actions. 13. The system of claim 12, wherein the policy neural network has been trained using supervised learning. 14. The system of claim 12, wherein the policy neural network has been trained using reinforcement learning. 15. The system of claim 1, wherein selecting actions to be performed by the agent to interact with the environment by traversing the state tree from the root node according to the edge data comprises: receiving an observation characterizing an in-tree state that is represented by an in-tree node that is between the root node and the leaf node; determining, for each edge from the in-tree node to another node, an adjusted action score for the edge based on the action score for the edge, the visit count for the edge, and the prior probability for the edge; and selecting an action represented by an edge having a highest adjusted action score as the action to be performed by the agent in response to the observation characterizing the in-tree state. 16. The system of claim 15, wherein the adjusted action score for each edge is the action score for each edge plus a bonus that is proportional to the prior probability for the edge but decays with repeated visits to encourage exploration. 17. The system of claim 1, the operations further comprising: receiving a current observation characterizing a current state of the environment; identifying a current node representing the current state; and selecting an action to be performed by the agent from edge data for outgoing edges from the current node. 18. The system of claim 17, wherein selecting an action to be performed by the agent from edge data for outgoing edges from the current node comprises: selecting the action from the actions represented by outgoing edges from the current that has the highest visit count. 19. The system of claim 17, wherein selecting an action to be performed by the agent from edge data for outgoing edges from the current node comprises: selecting the action from the actions represented by outgoing edges from the current that has the highest action score. 20. A method comprising: maintaining data representing a state tree of an environment being interacted with by an agent by performing actions selected from a set of possible actions, wherein the state tree comprises a plurality of nodes and a plurality of edges, wherein each node represents a respective state of the environment that has been encountered by the agent during interaction with the environment or with a simulated version of the environment, and wherein each edge connects a respective first node to a respective second node and represents an action that (i) was performed by the agent in response to an observation characterizing a first state represented by the respective first node and (ii) resulted in the environment or the simulated version of the environment transitioning into a second state represented by the respective second node; maintaining edge data, the edge data comprising, for each of the plurality of edges: (i) a respective action score for the action represented by the edge, (ii) a respective visit count that represents a number of times that the action represented by the edge has been performed by the agent in response to observations characterizing the respective first state represented by the respective first node for the edge, and (iii) a respective prior probability; receiving a root observation characterizing a root state of the environment that is represented by a root node of the plurality of nodes; selecting actions to be performed by the agent to interact with the environment or the simulated version of the environment by traversing the state tree from the root node according to the edge data until the environment or the simulated version of the environment reaches a leaf state represented by a leaf node in the state tree; and evaluating the leaf node by processing a leaf observation characterizing the leaf node using a value neural network that has been trained to process the leaf observation to generate a value score for the leaf state that represents a predicted long-term reward received as a result of the environment being in the leaf state. 21. One or more non-transitory storage media storing instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: maintaining data representing a state tree of an environment being interacted with by an agent by performing actions selected from a set of possible actions, wherein the state tree comprises a plurality of nodes and a plurality of edges, wherein each node represents a respective state of the environment that has been encountered by the agent during interaction with the environment or with a simulated version of the environment, and wherein each edge connects a respective first node to a respective second node and represents an action that (i) was performed by the agent in response to an observation characterizing a first state represented by the respective first node and (ii) resulted in the environment or the simulated version of the environment transitioning into a second state represented by the respective second node; maintaining edge data, the edge data comprising, for each of the plurality of edges: (i) a respective action score for the action represented by the edge, (ii) a respective visit count that represents a number of times that the action represented by the edge has been performed by the agent in response to observations characterizing the respective first state represented by the respective first node for the edge, and (iii) a respective prior probability; receiving a root observation characterizing a root state of the environment that is represented by a root node of the plurality of nodes; selecting actions to be performed by the agent to interact with the environment or the simulated version of the environment by traversing the state tree from the root node according to the edge data until the environment or the simulated version of the environment reaches a leaf state represented by a leaf node in the state tree; and evaluating the leaf node by processing a leaf observation characterizing the leaf node using a value neural network that has been trained to process the leaf observation to generate a value score for the leaf state that represents a predicted long-term reward received as a result of the environment being in the leaf state.