A computational system can include digital circuitry and analog circuitry, for instance a digital processor and a quantum processor. The quantum processor can operate as a sample generator providing samples. Samples can be employed by the digital processing in implementing various machine learning techniques. For example, the computational system can perform unsupervised learning over an input space, for example via a discrete variational auto-encoder, and attempting to maximize the log-likelihood of an observed dataset. Maximizing the log-likelihood of the observed dataset can include generating a hierarchical approximating posterior. Unsupervised learning can include generating samples of a prior distribution using the quantum processor. Generating samples using the quantum processor can include forming chains of qubits and representing discrete variables by chains.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A method for unsupervised learning over an input space comprising discrete or continuous variables, and at least a subset of a training dataset of samples of the respective variables, to attempt to identify a value of at least one parameter that increases a log-likelihood of at least the subset of the training dataset with respect to a model, the model expressible as a function of the at least one parameter, the method executed by circuitry including at least one processor, the method comprising; forming a first latent space comprising a plurality of random variables, the plurality of random variables comprising one or more discrete random variables; forming a second latent space comprising the first latent space and a set of supplementary continuous random variables; forming a first transforming distribution comprising a conditional distribution over the set of supplementary continuous random variables, conditioned on the one or more discrete random variables of the first latent space; forming an encoding distribution comprising an approximating posterior distribution over the first latent space, conditioned on the input space; forming a prior distribution over the first latent space; forming a decoding distribution comprising a conditional distribution over the input space conditioned on the set of supplementary continuous random variables; determining an ordered set of conditional cumulative distribution functions of the supplementary continuous random variables, each cumulative distribution function comprising functions of a full distribution of at least one of the one or more discrete random variables of the first latent space; determining an inversion of the ordered set of conditional cumulative distribution functions of the supplementary continuous random variables; constructing a first stochastic approximation to a lower bound on the log-likelihood of the at least a subset of a training dataset; constructing a second stochastic approximation to a gradient of the lower bound on the log-likelihood of at least the subset of the training dataset; and increasing the lower bound on the log-likelihood of at least the subset of the training dataset based at least in part on the gradient of the lower bound on the log-likelihood of at least the subset of the training dataset, wherein constructing a second stochastic approximation to a gradient of the lower bound includes approximating a gradient of at least a first part of the first stochastic approximation with respect to one or more parameters of the prior distribution over the first latent space using samples from the prior distribution, wherein approximating the gradient of at least a first part of the first stochastic approximation with respect to one or more parameters of the prior distribution over the first latent space using samples from the prior distribution includes at least one of generating a plurality of samples or causing a plurality of samples to be generated by a quantum processor comprising a plurality of qubits and a plurality of coupling devices providing communicative coupling between respective pairs of qubits, wherein at least one of generating a plurality of samples or causing a plurality of samples to be generated by a quantum processor includes: forming one or more chains, each chain comprising a respective subset of the plurality of qubits; and representing at least one of the one or more discrete random variables of the first latent space by a respective chain. 2. The method of claim 1 wherein forming one or more chains includes initiating a coupling strength of at least one coupling device, the coupling device selected to induce a correlation between a respective pair of qubits. 3. The method of claim 1 further comprising: determining an approximating posterior distribution over each of the first latent space and the second latent space, wherein the approximating posterior distribution is a hierarchical approximating posterior distribution comprising a plurality of levels of the hierarchy, and wherein at least one of generating a plurality of samples or causing a plurality of samples to be generated by a quantum processor further includes assigning a first qubit of each chain to a respective first level of the hierarchy. 4. The method of claim 3 wherein at least one of generating a plurality of samples or causing a plurality of samples to be generated by a quantum processor further includes assigning a second qubit of each chain to a respective second level of the hierarchy, wherein the second qubit of each chain is successively adjacent in the respective chain to the first qubit of the respective chain, and the likelihood of the second qubit of each chain for a given sample having the same value as the first qubit of the respective chain exceeds a predetermined threshold. 5. The method of claim 4 wherein at least one of generating a plurality of samples or causing a plurality of samples to be generated by a quantum processor further includes assigning a third qubit of each chain to a respective third level of the hierarchy, wherein the third qubit of each chain is successively adjacent in the respective chain to the second qubit of the respective chain, and the likelihood of the third qubit of each chain for a given sample having the same value as the second qubit of the respective chain exceeds the predetermined threshold. 6. The method of claim 4 wherein at least one of generating a plurality of samples or causing a plurality of samples to be generated by a quantum processor further includes assigning a third qubit of each chain to a respective third level of the hierarchy, wherein the third qubit of each chain is successively adjacent in the respective chain to the second qubit of the respective chain, and the likelihood of the third qubit of each chain for a given sample having the same value as the first qubit of the respective chain exceeds the predetermined threshold. 7. The method of claim 3 wherein the first qubit of each chain is at one end of the chain. 8. The method of claim 3 wherein the first qubit of each chain is in the interior of the chain, and wherein at least one of generating a plurality of samples or causing a plurality of samples to be generated by a quantum processor further includes assigning a second and a third qubit of each chain to a respective second level of the hierarchy, wherein the second and third qubits are both successively adjacent in the respective chain to the respective first qubit. 9. The method of claim 1 further comprising: determining an approximating posterior distribution over each of the first latent space and the second latent space, wherein the approximating posterior distribution is a hierarchical approximating posterior distribution comprising a plurality of levels of the hierarchy, wherein at least one of generating a plurality of samples or causing a plurality of samples to be generated by a quantum processor further includes assigning a single qubit of each chain to a respective level of the hierarchy. 10. The method of claim 1 further comprising: determining an approximating posterior distribution over each of the first latent space and the second latent space, wherein the approximating posterior distribution is a hierarchical approximating posterior distribution comprising a plurality of levels of the hierarchy, wherein at least one of generating a plurality of samples or causing a plurality of samples to be generated by a quantum processor further includes assigning a single qubit of each chain to a single level of the hierarchy. 11. The method of claim 1 further comprising: determining an approximating posterior distribution over each of the first latent space and the second latent space, wherein the approximating posterior distribution is a hierarchical approximating posterior distribution comprising a plurality of levels of the hierarchy, wherein at least one of generating a plurality of samples or causing a plurality of samples to be generated by a quantum processor further includes assigning a single qubit of each chain to a first level of the hierarchy, and remaining qubits of each chain to a second level of the hierarchy. 12. The method of claim 1 wherein increasing the lower bound on the log-likelihood of at least the subset of the training dataset based at least in part on the gradient of the lower bound on the log-likelihood of at least the subset of the training dataset includes increasing the lower bound on the log-likelihood of at least the subset of the training dataset using a method of gradient descent. 13. The method of claim 12 wherein increasing the lower bound on the log-likelihood of at least the subset of the training dataset using a method of gradient descent includes attempting to maximize the lower bound on the log-likelihood of at least the subset of the training dataset using a method of gradient descent. 14. The method of claim 1 where determining an ordered set of conditional cumulative distribution functions of the supplementary continuous random variables includes analytically determining an ordered set of conditional cumulative distribution functions of the supplementary continuous random variables. 15. The method of claim 1 wherein constructing a first stochastic approximation to the lower bound of the log-likelihood of the at least a subset of a training dataset includes: decomposing the first stochastic approximation to the lower bound into at least a first part comprising negative KL-divergence between the approximating posterior and the prior distribution over the first latent space, and a second part comprising an expectation, or at least a stochastic approximation to an expectation, with respect to the approximating posterior over the second latent space of the conditional log-likelihood of at least the subset of the training dataset under the decoding distribution. 16. The method of claim 1 wherein constructing a second stochastic approximation to the gradient of the lower bound further includes: determining the gradient of the second part of the first stochastic approximation by backpropagation; and determining a gradient of the first part of the first stochastic approximation with respect to parameters of the encoding distribution by backpropagation. 17. The method of claim 1 wherein a logarithm of the prior distribution is, to within a constant, a problem Hamiltonian of a quantum processor. 18. The method of claim 1, further comprising: determining an expectation with respect to the prior distribution from the plurality of samples. 19. The method of claim 18 wherein generating a plurality of samples or causing a plurality of samples to be generated by at least one quantum processor includes performing at least one post-processing operation on the plurality of samples. 20. The method of claim 18 wherein generating a plurality of samples or causing a plurality of samples to be generated by at least one quantum processor further includes: operating the at least one quantum processor as a sample generator to provide the plurality of samples from a probability distribution, wherein a shape of the probability distribution depends on a configuration of a number of programmable parameters for the at least one quantum processor, and wherein operating the at least one quantum processor as a sample generator comprises: programming the at least one quantum processor with a configuration of the number of programmable parameters for the at least one quantum processor, wherein the configuration of a number of programmable parameters corresponds to the probability distribution over the plurality of qubits of the at least one quantum processor; evolving the quantum processor; and reading out states for the qubits in plurality of qubits of the at least one quantum processor, wherein the states for the qubits in the plurality of qubits correspond to a sample from the probability distribution. 21. The method of claim 20 wherein operating the at least one quantum processor as a sample generator to provide the samples from a probability distribution includes operating the at least one quantum processor as a sample generator to provide the samples from a probability distribution wherein the shape of the probability distribution depends on at least one programmable qubit bias, and at least one programmable coupling strength of a coupling device providing communicative coupling between a respective pair of qubits. 22. The method of claim 1 wherein generating a plurality of samples or causing a plurality of samples to be generated by at least one quantum processor includes generating a plurality of samples or causing a plurality of samples to be generated by a restricted Boltzmann machine, the method further comprising determining the expectation with respect to the prior distribution from the plurality of samples. 23. The method of claim 1 wherein the set of supplementary continuous random variables comprises a plurality of continuous variables, and each one of the plurality of continuous variables is conditioned on a different respective one of the plurality of random variables. 24. The method of claim 1, further comprising: forming a second transforming distribution, wherein the input space comprises a plurality of input variables, and the second transforming distribution is conditioned on one or more of the plurality of input variables and at least one of the one or more discrete random variables. 25. A computational system, comprising: at least one processor; and at least one nontransitory processor-readable storage medium that stores at least one of processor-executable instructions or data which, when executed by the at least one processor cause the at least one processor to: form a first latent space comprising a plurality of random variables, the plurality of random variables comprising one or more discrete random variables; form a second latent space comprising the first latent space and a set of supplementary continuous random variables; form a first transforming distribution comprising a conditional distribution over the set of supplementary continuous random variables, conditioned on the one or more discrete random variables of the first latent space; form an encoding distribution comprising an approximating posterior distribution over the first latent space, conditioned on the input space; form a prior distribution over the first latent space; form a decoding distribution comprising a conditional distribution over the input space conditioned on the set of supplementary continuous random variables; determine an ordered set of conditional cumulative distribution functions of the supplementary continuous random variables, each cumulative distribution function comprising functions of a full distribution of at least one of the one or more discrete random variables of the first latent space; determine an inversion of the ordered set of conditional cumulative distribution functions of the supplementary continuous random variables; construct a first stochastic approximation to a lower bound on the log-likelihood of the at least a subset of a training dataset; construct a second stochastic approximation to a gradient of the lower bound on the log-likelihood of at least the subset of the training dataset; and increase the lower bound on the log-likelihood of at least the subset of the training dataset based at least in part on the gradient of the lower bound on the log-likelihood of at least the subset of the training dataset, wherein causing the at least one processor to construct a second stochastic approximation to a gradient of the lower bound includes causing the at least one processor to approximate a gradient of at least a first part of the first stochastic approximation with respect to one or more parameters of the prior distribution over the first latent space using samples from the prior distribution, wherein causing the at least one processor to approximate the gradient of at least a first part of the first stochastic approximation with respect to one or more parameters of the prior distribution over the first latent space using samples from the prior distribution includes at least one of causing the at least one processor to generate a plurality of samples or causing a quantum processor to generate a plurality of samples, the quantum processor comprising a plurality of qubits and a plurality of coupling devices providing communicative coupling between respective pairs of qubits, wherein at least one of causing the at least one processor to generate a plurality of samples or causing a quantum processor to generate a plurality of samples includes: form one or more chains, each chain comprising a respective subset of the plurality of qubits; and represent at least one of the one or more discrete random variables of the first latent space by a respective chain.