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Title: Wasserstein Barycenter Model Ensembling

Paper content:
In this paper we propose to perform model ensembling in a multiclass or a multilabel learning setting using Wasserstein (W.) barycenters.
Optimal transport metrics, such as the Wasserstein distance, allow incorporating semantic side information such as word embeddings.
Using W. barycenters to find the consensus between models allows us to balance confidence and semantics in finding the agreement between the models.
We show applications of Wasserstein ensembling in attribute-based classification, multilabel learning and image captioning generation.
These results show that the W. ensembling is a viable alternative to the basic geometric or arithmetic mean ensembling.
Model ensembling consists in combining many models into a stronger, more robust and more accurate model.
Ensembling is ubiquitous in machine learning and yields improved accuracies across multiple prediction tasks such as multi-class or multi-label classification.
For instance in deep learning, output layers of Deep Neural Networks(DNNs), such as softmaxes or sigmoids, are usually combined using a simple arithmetic or geometric mean.
The arithmetic mean rewards confidence of the models while the geometric means seeks the consensus across models.What is missing in the current approaches to models ensembling, is the ability to incorporate side information such as class relationships represented by a graph or via an embedding space.
For example a semantic class can be represented with a finite dimensional vector in a pretrained word embedding space such as GloVe BID28 .
The models' predictions can be seen as defining a distribution in this label space defined by word embeddings: if we denote p i to be the confidence of a model on a bin corresponding to a word having an embedding x i , the distribution on the label space is therefore p = i p i δ xi .
In order to find the consensus between many models predictions, we propose to achieve this consensus within this representation in the label space.
In contrast to arithmetic and geometric averaging, which are limited to the independent bins' confidence, this has the advantage of carrying the semantics to model averaging via the word embeddings.
More generally this semantic information can be encoded via cost a matrix C, where C ij encodes the dissimilarity between semantic classes i and j, and C defines a ground metric on the label space.To achieve this goal, we propose to combine model predictions via Wasserstein (W.) barycenters BID0 , which enables us to balance the confidence of the models and the semantic side information in finding a consensus between the models.
Wasserstein distances are a naturally good fit for such a task, since they are defined with respect to a ground metric in the label space of the models, which carry such semantic information.
Moreover they enable the possiblity of ensembling predictions defined on different label sets, since the Wasserstein distance allows to align and compare those different predictions.
Since their introduction in BID0 W. barycenter computations were facilitated by entropic regularization BID6 and iterative algorithms that rely on iterative Bregman projections BID2 .
Many applications have used W. barycenters in Natural Language Processing (NLP), clustering and graphics.
We show in this paper that W. barycenters are effective in model ensembling and in finding a semantic consensus, and can be applied to a wide range of problems in machine learning (Table 1) .The
paper is organized as follows: In Section 2 we revisit geometric and arithmetic means from a geometric viewpoint, showing that they are 2 and Kullback Leibler divergence KL (extended KL divergence) barycenters respectively. We
give a brief overview of optimal transport metric and W. barycenters in Section 3.
We highlight the advantages of W. barycenter ensembling in terms of semantic smoothness and diversity in Section 4.
Related work on W. barycenters in Machine learning are presented in Section 5.
Finally we show applications of Wasserstein ensembling on attribute based classification, multi-label learning and image captioning in Section 6.
We showed in this paper that W. barycenters are effective in model ensembling in machine learning.
In the unbalanced case we showed their effectiveness in attribute based classification, as well as in improving the accuracy of multi-label classification.
In the balanced case, we showed that they promote diversity and improve natural language generation by incorporating the knowledge of synonyms or word embeddings.
Table 8 : Sample output (top 20 words) of barycenter for different similarity matrices K based on GloVe (columns titles denote the distance of K from identity K − I F and corresponding .).
Each column shows a word and its corresponding probability over the vocabulary.
Note that the last column coincides with the output from geometric mean.
Table 8 shows the effect of entropic regularization ε on the resulting distribution of the words of W. barycenter using GloVe embedding matrix.
As K moves closer to the identity matrix, the entropy of barycenter decreases, leading to outputs that are close/identical to the geometric mean.
On the other hand, with a large entropic regularization, matrix K moves away from identity, becoming an uninformative matrix of all 1's.
This eventually leads to a uniform distribution which spreads the probability mass equally across all the words.
This can be also visualized with a histogram in Figure 5 , where the histograms on the bottom represent distributions that are close to uniform, which can be considered as failure cases of W. barycenter, since the image captioner in this case can only generate meaningless, gibberish captions.

assistant: text: we propose to use Wasserstein barycenters for semantic model ensembling