Paper:
Learning to align multiple datasets is an important problem with many applications, and it is especially useful when we need to integrate multiple experiments or correct for confounding. Optimal transport (OT) is a principled approach to align datasets, but a key challenge in applying OT is that we need to specify a transport cost function that accurately captures how the two datasets are related. Reliable cost functions are typically not available and practitioners often resort to using hand-crafted or Euclidean cost even if it may not be appropriate. In this work, we investigate how to learn the cost function using a small amount of side information which is often available. The side information we consider captures subset correspondence-i.e. certain subsets of points in the two data sets are known to be related. For example, we may have some images labeled as cars in both datasets; or we may have a common annotated cell type in single-cell data from two batches. We develop an end-to-end optimizer (OT-SI) that differentiates through the Sinkhorn algorithm and effectively learns the suitable cost function from side information. On systematic experiments in images, marriage-matching and single-cell RNA-seq, our method substantially outperform state-of-the-art benchmarks.

Review:
The authors consider the alignment problem for multiple datasets with side information via entropic optimal transport (Sinkhorn). The authors formulate it as a transport cost learning in optimal transport framework with constraints giving by side information. Empirically, the authors illustrated the effectiveness of the proposed approach over state-of-the-art on several applications (e.g. single-cell RNA-seq, marriage-matching, MNIST with its perturbed one. 
The paper show good results of the proposed method for dataset alignment on several applications. However, the motivation of the dataset alignment seems quite weak (e.g. it is unclear the advantage for jointly analysis when one merge datasets by alignment). The side information is not clearly easy to obtain in practice, since it simply enforces the constraints on the transportation polytope (i.e. how one knows that which subset of a given class C_A on dataset A is matched with a certain subset of another class C_B on dataset B, it seems easy to obtain the side information about matching over the whole class C_A with C_B, and it seems nontrivial to obtain the subset matching). Additionally, some important information about the proposed method is not in the main text but in the appendix which makes the paper hard to read without checking appendix constantly (especially the main proposed method). Overall, I lean on the negative side.
Below are some of my concerns:
1) As stated above, I am curious how one can obtain the side information about the subset matching? For example, in Figure 1, it seems trivial to have a side information about the whole class matching and quite nontrivial to obtain a matching for subsets of any given class.
2) The side information enforces the constraints on the feasible set of the transportation polytope. It seems to me that one can solve the variant of optimal transport (or Sinkhorn) with those extra constraints directly, it is unclear to me how this kind of side information can navigate the transport cost learning. Moreover, given a new pairs of input, the proposed method needs to optimize the transport cost again which make the benefits to learn to transport cost quite limited. Overall, a trivial baseline for the proposed method is to solve directly optimal transport with those extra constrains (from side information) directly (maybe with the Euclidean cost metric), but not the optimal transport without extra constraints as in the current setting. 
3) The authors may need to consider to reorganize the presentation. Much important information of the proposed method is not in the main text, but in the appendix which make the paper hard to follow without checking the appendix constantly. The derivation of gradient in line 6 of Algorithm 1 is not trivial. It is better if the authors give more detail information and discussion. I am quite confused about the idea to back propagate the "gradient" through Sinkhorn-Knopp iterations?
4) It seems that the complexity of gradient update of Algorithm 1 is quite high. The authors should give a comparison about time consumption in the experiments (besides the accuracy matching)
5) It seems that there is nothing in the code sharing folder?