Paper:
Motivated by the need for higher order gradients in multi-agent reinforcement learning and meta-learning, this paper studies the construction of baselines for second order Monte Carlo gradient estimators in order to reduce the sample variance. Following the construction of a stochastic computation graph (SCG), the Infinitely Differentiable Monte-Carlo Estimator (DiCE) can generate correct estimates of arbitrary order gradients through differentiation. However, a baseline term that serves as a control variate for reducing variance is currently provided only for first order gradient estimation, limiting the utility of higher-order gradient estimates. To improve the sample efficiency of DiCE, we propose a new baseline term for higher order gradient estimation. This term may be easily included in the objective, and produces unbiased variance-reduced estimators under (automatic) differentiation, without affecting the estimate of the objective itself or of the first order gradient. Importantly it reuses the same baseline function (e.g. the state-value function in reinforcement learning) already used for the first order baseline. We provide theoretical analysis and numerical evaluations of our baseline term, which demonstrate that it can dramatically reduce the variance of second order gradient estimators produced by DiCE. This computational tool can be easily used to estimate second order gradients with unprecedented efficiency wherever automatic differentiation is utilised, and has the potential to unlock applications of higher order gradients in reinforcement learning and meta-learning.

Review:
Overview: 
This nicely written paper contributes a useful variance reduction baseline to make the recent formalism of the DiCE estimator more practical in application. I assess the novelty and scale of the current contribution as too low for publication at ICLR. Also, the paper includes a few incorrect assertions regarding the control variate framework as well as action-dependent baselines in reinforcement learning. Such issues reduce the value of the contribution in its current form and may contribute to ongoing misunderstandings of the control variate framework and action-dependent baselines in RL, to the detriment of variance reduction techniques in machine learning. I do not recommend publication at this time.
Pros:
The paper is well written modulo the issues discussed below. It strikes me as a valuable workshop contribution once the errors are addressed, but it lacks enough novelty for the main conference track.
Issues:
* (p.5) "R_w and b_w are positively correlated by design, as they should be for variance reduction of the first order gradients."
This statement is not true in general. Intuitively, a control variate reduces variance because when a single estimate of an expectation of a function diverges from its true value according to some delta, then, with high probability, some function strongly correlated with that function will also diverge with a similar delta. Such a delta might be positive or negative, so long as the error may be appropriately modeled as drawn from some symmetric distribution (i.e. is Gaussian).
Control variates are often estimated with an optimal scaling constant that depends on the covariance of the original function and its control variate. Due to the dependence on the covariance, the scaling constant flips sign as appropriate in order reduce variance for any delta. For more information, see the chapter on variance reduction and subsection on control variates in Sheldon Ross's textbook "Simulation."
The fact that a control variate appears to work despite this is not surprising. Biased and suboptimal unbiased gradient estimators have been shown to work well for reasons not fully explored in the literature yet. See, for example, Tucker et al.'s "Mirage of Action-Dependent Baselines", https://arxiv.org/abs/1802.10031.
Since the authors claim on page 6 that the baseline is positively correlated by design, this misunderstanding of the control variate framework appears to be baked into the baseline itself. I recommend the authors look into adaptively estimating an optimal scale for the baseline using a rolling estimator of the covariance and variance to fix this issue. See the Ross book cited above for full derivation of this optimal scale.
* The second error is a mischaracterization of the use and utility of action-dependent baselines for RL problems, on page 6: "We choose the baseline ... to be a function of state ... it must be independent of the action ...." and "it is essential to exclude the current action ... because the baselines ... must be independent of the action ... to remain unbiased." In the past year, a slew of papers have presented techniques for the use of action-dependent baselines, with mixed results (see the Mirage paper just cited), including two of the papers the authors cited.
Cons
* Much of paper revises the DiCE estimator results, arguing for and explaining again those results rather than referring to them as a citation. 
* I assess the novelty of proposed contribution as too low for publication. The baseline is an extension of the same method used in the original paper, and does not generalize past the second order gradient, making the promising formalism of the DiCE estimator as infinitely differentiable still unrealizable in practice.
* The experiments are practically identical to the DiCE estimator paper, also reducing the novelty and contribution of the paper.
*EDIT: 
I thank the authors for a careful point-by-point comparison of our disagreements on this paper so that we may continue the discussion. However, none of the points I identified were addressed, and so I maintain my original score and urge against publication. In their rebuttal, the authors have defended errors and misrepresentations in the original submission, and so I provide a detailed response to each of the numbered issues below:
(1) I acknowledge that it is common to set c=1 in experiments. This is not the same as the misstatements I cited, verbatim, in the paper that suggest this is required for variance reduction. My aim in identifying these mistakes is not to shame the authors (they appear to simply be typos) but simply to ensure that future work in this area begins with a correct understanding of the theory. I request again that the authors revise the cited lines that incorrectly state the reliance of a control variate on positive correlation. It is not enough to state that "everyone knows" what is meant when the actual claim is misleading.
(2) Without more empirical investigation, the authors' new claim that a strictly state-value-function baseline is a strength rather than a weakness cannot be evaluated. This may be the case, and I would welcome some set of experiments that establish this empirical claim by comparing against state-action-dependent baselines. The authors appear to believe that state-action-dependent baselines are never effective in reducing variance, and this is perhaps the central error in the paper that should be addressed. See response (3). Were the authors to fix this, they would necessarily compare against state-action-dependent baselines, which would be of great value for the community at large in settling this open issue.
(3) Action-dependent baselines have not been shown to be ineffective. I wish to strongly emphasize that this is not the conclusion of the Mirage paper, and the claim repeated in the authors' response (3) has not been validated empirically or analytically, and does not represent the state of variance reduction in reinforcement learning as of this note. I repeat a few key arguments from the Mirage paper in an attempt to dispel the authors' repeated misinterpretation of the paper.
The variance of the policy gradient estimator, subject to a baseline "phi," is decomposed using the Law of Total Variance in Eq (3) of the Mirage paper. This decomposition identifies a non-zero contribution from "phi(a,s)", the (adaptive or non-adaptive) baseline. The Mirage paper analyzes under what conditions such a contribution is expected to be non-negligible. Quoting from the paper:
"We expect this to be the case when single actions have a large effect on the overall discounted
return (e.g., in a Cliffworld domain, where a single action could cause the agent to fall of the cliff and suffer a large negative reward)."
Please see Sec. 3, "Policy Gradient Variance Decomposition" of the Mirage paper for further details.
The Mirage paper does indeed cast reasonable doubt on subsets of a few papers' experiments, and shows that the strong claim, mistakenly made by these papers, that state-action-dependence is always required for an adaptive control variate to reduce variance over state dependence, is not true. 
It should be clear from the discussion of the paper to this point that this does _not_ imply the even stronger claim in "A Better Second Order Baseline" that action dependence is never effective and should no longer be considered as a means to reduce variance from a practitioner's point of view. Such a misinterpretation should not be legitimized through publication, as it will muddy the waters in future research. I again urge the authors to remove this mistake from the paper.
(4) I acknowledge the efforts of the authors to ensure that adequate background is provided for readers. This is a thorny issue, and it is difficult to balance in any work. Since this material represents a sizeable chunk of the paper and is nearly identical to existing published work, it leads me to lower the score for novelty of contribution simply by that fact. Perhaps the authors could have considered placing the extensive background materials in the appendix and instead summarizing them briefly in the body of the paper, leaving more room for discussion and experimental validation beyond the synthetic cases already studied in the DiCE paper.
(5), (6) In my review I provided specific, objective criteria by which I have assessed the novelty of this paper: the lack of original written material, and the nearly identical experiments to the DiCE paper. As I noted in response (4) above, this reduces space for further analysis and experimentation.