user: text: Write a concise one-sentence TLDR summary of the scientific paper content below.

Title: The Differentiable Cross-Entropy Method

Paper content:
We study the Cross-Entropy Method (CEM) for the non-convex optimization of a continuous and parameterized objective function and introduce a differentiable variant (DCEM) that enables us to differentiate the output of CEM with respect to the objective function's parameters.
In the machine learning setting this brings CEM inside of the end-to-end learning pipeline in cases this has otherwise been impossible.
We show applications in a synthetic energy-based structured prediction task and in non-convex continuous control.
In the control setting we show on the simulated cheetah and walker tasks that we can embed their optimal action sequences with DCEM and then use policy optimization to fine-tune components of the controller as a step towards combining model-based and model-free RL.
Recent work in the machine learning community has shown how optimization procedures can create new building-blocks for the end-to-end machine learning pipeline (Gould et al., 2016; Johnson et al., 2016; Domke, 2012; Metz et al., 2016; Finn et al., 2017; Belanger et al., 2017; Rusu et al., 2018; Srinivas et al., 2018; Amos et al., 2018) .
In this paper we focus on the setting of optimizing an unconstrained, non-convex, and continuous objective function f θ (x) : R n × Θ → R asx = arg min x f θ (x), where f is parameterized by θ ∈ Θ and has inputs x ∈ R n .
If it exists, some (sub-)derivative ∇ θx is useful in the machine learning setting to make the output of the optimization procedure end-to-end learnable.
For example, θ could parameterize a predictive model that is generating potential outcomes conditional on x happening that you want to optimize over.
End-to-end learning in these settings can be done by defining a loss function L on top ofx and taking gradient steps ∇ θ L. If f θ were convex this gradient is easy to analyze and compute when it exists and is unique (Gould et al., 2016; Johnson et al., 2016; .
Unfortunately analyzing and computing a "derivative" through the non-convex arg min here is not as easy and is challenging in theory and practice.
No such derivative may exist in theory, it might not be unique, and even if it uniquely exists, the numerical solver being used to compute the solution may not find a global or even local optimum of f .
One promising direction to sidestep these issues is to approximate the arg min operation with an explicit optimization procedure that is interpreted as just another compute graph and unrolled through.
This is most commonly done with gradient descent as in Domke (2012) ; Metz et al. (2016) ; Finn et al. (2017) ; Belanger et al. (2017) ; Rusu et al. (2018) ; Srinivas et al. (2018) ; Foerster et al. (2018) .
This approximation adds significant definition and structure to an otherwise extremely ill-defined desiderata at the cost of biasing the gradients and enabling the learning procedure to over-fit to the hyper-parameters of the optimization algorithm, such as the number of gradient steps or the learning rate.
In this paper we show that the Cross-Entropy Method (CEM) (De Boer et al., 2005 ) is a reasonable alternative to unrolling gradient descent for approximating the derivative through an unconstrained, non-convex, and continuous arg min.
CEM for optimization is a zeroth-order optimizer and works by generating a sequence of samples from the objective function.
We show a simple and computationally negligible way of making CEM differentiable that we call DCEM by using the smooth top-k operation from Amos et al. (2019) .
This also brings CEM into the end-to-end learning process in cases where there is otherwise a disconnection between the objective that is being learned and the objective that is induced by deploying CEM on top of those models.
We first quickly study DCEM in a simple non-convex energy-based learning setting for regression.
We contrast using unrolled gradient descent and DCEM for optimizing over a SPEN (Belanger & McCallum, 2016) .
We show that unrolling through gradient descent in this setting over-fits to the number of gradient steps taken and that DCEM generates a more reasonable energy surface.
Our main application focus is on using DCEM in the context of non-convex continuous control.
This setting is especially interesting as vanilla CEM is the state-of-the-art method for solving the control optimization problem with neural network transition dynamics as in Chua et al. (2018) ; Hafner et al. (2018) .
We show that DCEM is useful for embedding action sequences into a lower-dimensional space to make solving the control optimization process significantly less computationally and memory expensive.
This gives us a controller that induces a differentiable policy class parameterized by the model-based components.
We then use PPO (Schulman et al., 2017) to fine-tune the modelbased components, demonstrating that it is possible to use standard policy learning for model-based RL in addition to just doing maximum-likelihood fitting to observed trajectories.
We have laid the foundations for differentiating through the cross-entropy method and have brought CEM into the end-to-end learning pipeline.
Beyond further explorations in the energy-based learning and control contexts we showed here, DCEM can be used anywhere gradient descent is unrolled.
We find this especially promising for meta-learning, potentially building on LEO (Rusu et al., 2018) .
Inspired by DCEM, other more powerful sampling-based optimizers could be made differentiable in the same way, potentially optimizers that leverage gradient-based information in the inner optimization steps (Sekhon & Mebane, 1998; Theodorou et al., 2010; Stulp & Sigaud, 2012; Maheswaranathan et al., 2018) or by also learning the hyper-parameters of structured optimizers (Li & Malik, 2016; Volpp et al., 2019; Chen et al., 2017) .

assistant: text: DCEM learns latent domains for optimization problems and helps bridge the gap between model-based and model-free RL --- we create a differentiable controller and fine-tune parts of it with PPO