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- ShortScience.org is a platform for post-publication discussion aiming to improve accessibility and reproducibility of research ideas.
- The website has 1583 public summaries, mostly in machine learning, written by the community and organized by paper, conference, and year.
- Reading summaries of papers is useful to obtain the perspective and insight of another reader, why they liked or disliked it, and their attempt to demystify complicated sections.
- Also, writing summaries is a good exercise to understand the content of a paper because you are forced to challenge your assumptions when explaining it.
- Finally, you can keep up to date with the flood of research by reading the latest summaries on our Twitter and Facebook pages.

Stabilizing Off-Policy Q-Learning via Bootstrapping Error Reduction

Kumar, Aviral and Fu, Justin and Soh, Matthew and Tucker, George and Levine, Sergey

Neural Information Processing Systems Conference - 2019 via Local Bibsonomy

Keywords: dblp

Kumar, Aviral and Fu, Justin and Soh, Matthew and Tucker, George and Levine, Sergey

Neural Information Processing Systems Conference - 2019 via Local Bibsonomy

Keywords: dblp

[link]
Kumar et al. propose an algorithm to learn in batch reinforcement learning (RL), a setting where an agent learns purely form a fixed batch of data, $B$, without any interactions with the environments. The data in the batch is collected according to a batch policy $\pi_b$. Whereas most previous methods (like BCQ) constrain the learned policy to stay close to the behavior policy, Kumar et al. propose bootstrapping error accumulation reduction (BEAR), which constrains the newly learned policy to place some probability mass on every non negligible action. The difference is illustrated in the picture from the BEAR blog post: https://i.imgur.com/zUw7XNt.png The behavior policy is in both images the dotted red line, the left image shows the policy matching where the algorithm is constrained to the purple choices, while the right image shows the support matching. **Theoretical Contribution:** The paper analysis formally how the use of out-of-distribution actions to compute the target in the Bellman equation influences the back-propagated error. Firstly a distribution constrained backup operator is defined as $T^{\Pi}Q(s,a) = \mathbb{E}[R(s,a) + \gamma \max_{\pi \in \Pi} \mathbb{E}_{P(s' \vert s,a)} V(s')]$ and $V(s) = \max_{\pi \in \Pi} \mathbb{E}_{\pi}[Q(s,a)]$ which considers only policies $\pi \in \Pi$. It is possible that the optimal policy $\pi^*$ is not contained in the policy set $\Pi$, thus there is a suboptimallity constant $\alpha (\Pi) = \max_{s,a} \vert \mathcal{T}^{\Pi}Q^{*}(s,a) - \mathcal{T}Q^{*}(s,a) ]\vert $ which captures how far $\pi^{*}$ is from $\Pi$. Letting $P^{\pi_i}$ be the transition-matrix when following policy $\pi_i$, $\rho_0$ the state marginal distribution of the training data in the batch and $\pi_1, \dots, \pi_k \in \Pi $. The error analysis relies upon a concentrability assumption $\rho_0 P^{\pi_1} \dots P^{\pi_k} \leq c(k)\mu(s)$, with $\mu(s)$ the state marginal. Note that $c(k)$ might be infinite if the support of $\Pi$ is not contained in the state marginal of the batch. Using the coefficients $c(k)$ a concentrability coefficient is defined as: $C(\Pi) = (1-\gamma)^2\sum_{k=1}^{\infty}k \gamma^{k-1}c(k).$ The concentrability takes values between 1 und $\infty$, where 1 corresponds to the case that the batch data were collected by $\pi$ and $\Pi = \{\pi\}$ and $\infty$ to cases where $\Pi$ has support outside of $\pi$. Combining this Kumar et a. get a bound of the Bellman error for distribution constrained value iteration with the constrained Bellman operator $T^{\Pi}$: $\lim_{k \rightarrow \infty} \mathbb{E}_{\rho_0}[\vert V^{\pi_k}(s)- V^{*}(s)] \leq \frac{\gamma}{(1-\gamma^2)} [C(\Pi) \mathbb{E}_{\mu}[\max_{\pi \in \Pi}\mathbb{E}_{\pi}[\delta(s,a)] + \frac{1-\gamma}{\gamma}\alpha(\Pi) ] ]$, where $\delta(s,a)$ is the Bellman error. This presents the inherent batch RL trade-off between keeping policies close to the behavior policy of the batch (captured by $C(\Pi)$ and keeping $\Pi$ sufficiently large (captured by $\alpha(\Pi)$). It is finally proposed to use support sets to construct $\Pi$, that is $\Pi_{\epsilon} = \{\pi \vert \pi(a \vert s)=0 \text{ whenever } \beta(a \vert s) < \epsilon \}$. This amounts to the set of all policies that place probability on all non-negligible actions of the behavior policy. For this particular choice of $\Pi = \Pi_{\epsilon}$ the concentrability coefficient can be bounded. **Algorithm**: The algorithm has an actor critic style, where the Q-value to update the policy is taken to be the minimum over the ensemble. The support constraint to place at least some probability mass on every non negligible action from the batch is enforced via sampled MMD. The proposed algorithm is a member of the policy regularized algorithms as the policy is updated to optimize: $\pi_{\Phi} = \max_{\pi} \mathbb{E}_{s \sim B} \mathbb{E}_{a \sim \pi(\cdot \vert s)} [min_{j = 1 \dots, k} Q_j(s,a)] s.t. \mathbb{E}_{s \sim B}[MMD(D(s), \pi(\cdot \vert s))] \leq \epsilon$ The Bellman target to update the Q-functions is computed as the convex combination of minimum and maximum of the ensemble. **Experiments** The experiments use the Mujoco environments Halfcheetah, Walker, Hopper and Ant. Three scenarios of batch collection, always consisting of 1Mio. samples, are considered: - completely random behavior policy - partially trained behavior policy - optimal policy as behavior policy The experiments confirm that BEAR outperforms other off-policy methods like BCQ or KL-control. The ablations show further that the choice of MMD is crucial as it is sometimes on par and sometimes substantially better than choosing KL-divergence. |

Distributional Smoothing with Virtual Adversarial Training

Takeru Miyato and Shin-ichi Maeda and Masanori Koyama and Ken Nakae and Shin Ishii

arXiv e-Print archive - 2015 via Local arXiv

Keywords: stat.ML, cs.LG

**First published:** 2015/07/02 (7 years ago)

**Abstract:** We propose local distributional smoothness (LDS), a new notion of smoothness
for statistical model that can be used as a regularization term to promote the
smoothness of the model distribution. We named the LDS based regularization as
virtual adversarial training (VAT). The LDS of a model at an input datapoint is
defined as the KL-divergence based robustness of the model distribution against
local perturbation around the datapoint. VAT resembles adversarial training,
but distinguishes itself in that it determines the adversarial direction from
the model distribution alone without using the label information, making it
applicable to semi-supervised learning. The computational cost for VAT is
relatively low. For neural network, the approximated gradient of the LDS can be
computed with no more than three pairs of forward and back propagations. When
we applied our technique to supervised and semi-supervised learning for the
MNIST dataset, it outperformed all the training methods other than the current
state of the art method, which is based on a highly advanced generative model.
We also applied our method to SVHN and NORB, and confirmed our method's
superior performance over the current state of the art semi-supervised method
applied to these datasets.
more
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Takeru Miyato and Shin-ichi Maeda and Masanori Koyama and Ken Nakae and Shin Ishii

arXiv e-Print archive - 2015 via Local arXiv

Keywords: stat.ML, cs.LG

[link]
Miyato et al. propose distributional smoothing (or virtual adversarial training) as defense against adversarial examples. However, I think that both terms do not give a good intuition of what is actually done. Essentially, a regularization term is introduced. Letting $p(y|x,\theta)$ be the learned model, the regularizer is expressed as $\text{KL}(p(y|x,\theta)|p(y|x+r,\theta)$ where $r$ is the perturbation that maximizes the Kullback-Leibler divergence above, i.e. $r = \arg\max_r \{\text{KL}(p(y|x,\theta)|p(y|x+r,\theta) | \|r\|_2 \leq \epsilon\}$ with hyper-parameter $\epsilon$. Essentially, the regularizer is supposed to “simulate” adversarial training – thus, the method is also called virtual adversarial training. The discussed implementation, however, is somewhat cumbersome. In particular, $r$ cannot be computed using first-order methods as the gradient of $\text{KL}$ is $0$ for $r = 0$. So a second-order method is used – for which the Hessian needs to be approximated and the corresponding eigenvectors need to be computed. For me it is unclear why $r$ cannot be initialized randomly to solve this issue … Then, the derivative of the regularizer needs to be computed during training. Here, the authors make several simplifications (such as fixing $\theta$ in the first part of the Kullback-Leibler divergence and ignoring the derivative of $r$ w.r.t. $\theta$). Overall, however, I like the idea of “virtual” adversarial training as it avoids the need of explicitly using attacks during training to craft adversarial examples. Then, the trained model is often robust against the chosen attacks, but new adversarial examples can be found easily through novel attacks. Also view this summary at [davidstutz.de](https://davidstutz.de/category/reading/). |

What is being transferred in transfer learning?

Neyshabur, Behnam and Sedghi, Hanie and Zhang, Chiyuan

arXiv e-Print archive - 2020 via Local Bibsonomy

Keywords: dblp

Neyshabur, Behnam and Sedghi, Hanie and Zhang, Chiyuan

arXiv e-Print archive - 2020 via Local Bibsonomy

Keywords: dblp

[link]
This is an interesting - and refreshing - paper, in that, instead of trying to go all-in on a particular theoretical point, the authors instead run a battery of empirical investigations, all centered around the question of how to explain what happens to make transfer learning work. The experiments don't all line up to support a single point, but they do illustrate different interesting facets of the transfer process. - An initial experiment tries to understand how much of the performance of fine-tuned models can be explained by (higher-level, and thus larger-scale) features, and how much is driven by lower level (and thus smaller-scale) image statistics. To start with, the authors compare the transfer performance from ImageNet onto three different datasets - clip art, sketches, and real images. As expected, transfer performance is highest with real datasets, which are the most similar to training domain. However, there still *is* positive transfer in terms of final performance across all domains, as well as benefit in optimization speed. - To try to further tease out the difference between the transfer benefits of high and low-level features, the authors run an experiment where blocks of pixels are shuffled around within the image on downstream tasks . The larger the size of the blocks being shuffled, the more that large-scale features of the image are preserved. As predicted, accuracy drops dramatically when pixel block size is small, for both randomly initialized and pretrained models. In addition, the relative value added by pretraining drops, for all datasets except quickdraw (the dataset of sketches). This suggests that in most datasets, the value brought by fine-tuning was mostly concentrated in large-scale features. One interesting tangent of this experiment was the examination of optimization speed (in the form of mean training accuracy over initial epochs). Even at block sizes too small for pretraining to offer a benefit to final accuracy, it did still contribute to faster training. (See transparent bars in right-hand plot below) https://i.imgur.com/Y8sO1da.png - On a somewhat different front, the authors look into how similar pretrained + finetuned models are to one another, compared to models trained on the same dataset from random initializations. First, they look at a measure of feature similarity, and find that the features learned by two pretrained networks are more similar to each other than a pretrained network is to a randomly initalized network, and also more than two randomly initialized networks are to one another. Randomly initialized networks are closest to one another in their final-layer features, but this is still a multiple of 4 or 5 less than the similarity between the pretrained networks - Looking at things from the perspective of optimization, the paper measures how much performance drops when you linearly interpolate between different solutions found by both randomly initialized and pretrained networks. For randomly initialized networks, interpolation requires traversing a region where test accuracy drops to 0%. However, for pretrained networks, this isn't the case, with test accuracy staying high throughout. This suggests that pretraining gets networks into a basin of the loss landscape, and that future training stays within that basin. There were also some experiments on module criticality that I believe were in a similar vein to these, but which I didn't fully follow - Finally, the paper looks at the relationship between accuracy on the original pretraining task and both accuracy and optimization speed on the downstream task. They find that higher original-task accuracy moves in the same direction as higher downstream-task accuracy, though this is less true when the downstream task is less related (as with quickdraw). Perhaps more interestingly, they find that the benefits of transfer to optimization speed happen and plateau quite early in training. Clip Art and Real transfer tasks are much more similar in the optimization speed benefits they get form ImageNet training, where on the accuracy front, the real did dramatically better. https://i.imgur.com/jBCJcLc.png While there's a lot to dig into in these results overall, the things I think are most interesting are the reinforcing of the idea that even very random and noisy pretraining can be beneficial to optimization speed (this seems reminiscent of another paper I read from this year's NeurIPS, examining why pretraining on random labels can help downstream training), and the observation that pretraining deposits weights in a low-loss bucket, from which they can learn more efficiently (though, perhaps, if the task is too divergent from the pretraining task, this difficulty in leaving the basin becomes a disadvantage). This feels consistent with some work in the Lottery Ticket Hypothesis, which has recently suggested that, after a short duration of training, you can rewind a network to a checkpoint saved after that duration, and be successfully able to train to low loss again. |

Not All Unlabeled Data are Equal: Learning to Weight Data in Semi-supervised Learning

Ren, Zhongzheng and Yeh, Raymond A. and Schwing, Alexander G.

- 2020 via Local Bibsonomy

Keywords: dataset, semi-supervised, machine-learning, data, 2020

Ren, Zhongzheng and Yeh, Raymond A. and Schwing, Alexander G.

- 2020 via Local Bibsonomy

Keywords: dataset, semi-supervised, machine-learning, data, 2020

[link]
This paper argues that, in semi-supervised learning, it's suboptimal to use the same weight for all examples (as happens implicitly, when the unsupervised component of the loss for each example is just added together directly. Instead, it tries to learn weights for each specific data example, through a meta-learning-esque process. The form of semi-supervised learning being discussed here is label-based consistency loss, where a labeled image is augmented and run through the current version of the model, and the model is optimized to try to induce the same loss for the augmented image as the unaugmented one. The premise of the authors argument for learning per-example weights is that, ideally, you would enforce consistency loss less on examples where a model was unconfident in its label prediction for an unlabeled example. As a way to solve this, the authors suggest learning a vector of parameters - one for each example in the dataset - where element i in the vector is a weight for element i of the dataset, in the summed-up unsupervised loss. They do this via a two-step process, where first they optimize the parameters of the network given the example weights, and then the optimize the example weights themselves. To optimize example weights, they calculate a gradient of those weights on the post-training validation loss, which requires backpropogating through the optimization process (to determine how different weights might have produced a different gradient, which might in turn have produced better validation loss). This requires calculating the inverse Hessian (second derivative matrix of the loss), which is, generally speaking, a quite costly operation for huge-parameter nets. To lessen this cost, they pretend that only the final layer of weights in the network are being optimized, and so only calculate the Hessian with respect to those weights. They also try to minimize cost by only updating the example weights for the examples that were used during the previous update step, since, presumably those were the only ones we have enough information to upweight or downweight. With this model, the authors achieve modest improvements - performance comparable to or within-error-bounds better than the current state of the art, FixMatch. Overall, I find this paper a little baffling. It's just a crazy amount of effort to throw into something that is a minor improvement. A few issues I have with the approach: - They don't seem to have benchmarked against the simpler baseline of some inverse of using Dropout-estimated uncertainty as the weight on examples, which would, presumably, more directly capture the property of "is my model unsure of its prediction on this unlabeled example" - If the presumed need for this is the lack of certainty of the model, that's a non-stationary problem that's going to change throughout the course of training, and so I'd worry that you're basically taking steps in the direction of a moving target - Despite using techniques rooted in meta-learning, it doesn't seem like this models learns anything generalizable - it's learning index-based weights on specific examples, which doesn't give it anything useful it can do with some new data point it finds that it wasn't specifically trained on Given that, I think I'd need to see a much stronger case for dramatic performance benefits for something like this to seem like it was worth the increase in complexity (not to mention computation, even with the optimized Hessian scheme) |

Mask R-CNN

He, Kaiming and Gkioxari, Georgia and Dollár, Piotr and Girshick, Ross B.

arXiv e-Print archive - 2017 via Local Bibsonomy

Keywords: dblp

He, Kaiming and Gkioxari, Georgia and Dollár, Piotr and Girshick, Ross B.

arXiv e-Print archive - 2017 via Local Bibsonomy

Keywords: dblp

[link]
Mask RCNN takes off from where Faster RCNN left, with some augmentations aimed at bettering instance segmentation (which was out of scope for FRCNN). Instance segmentation was achieved remarkably well in *DeepMask* , *SharpMask* and later *Feature Pyramid Networks* (FPN). Faster RCNN was not designed for pixel-to-pixel alignment between network inputs and outputs. This is most evident in how RoIPool , the de facto core operation for attending to instances, performs coarse spatial quantization for feature extraction. Mask RCNN fixes that by introducing RoIAlign in place of RoIPool. #### Methodology Mask RCNN retains most of the architecture of Faster RCNN. It adds the a third branch for segmentation. The third branch takes the output from RoIAlign layer and predicts binary class masks for each class. ##### Major Changes and intutions **Mask prediction** Mask prediction segmentation predicts a binary mask for each RoI using fully convolution - and the stark difference being usage of *sigmoid* activation for predicting final mask instead of *softmax*, implies masks don't compete with each other. This *decouples* segmentation from classification. The class prediction branch is used for class prediction and for calculating loss, the mask of predicted loss is used calculating Lmask. Also, they show that a single class agnostic mask prediction works almost as effective as separate mask for each class, thereby supporting their method of decoupling classification from segmentation **RoIAlign** RoIPool first quantizes a floating-number RoI to the discrete granularity of the feature map, this quantized RoI is then subdivided into spatial bins which are themselves quantized, and finally feature values covered by each bin are aggregated (usually by max pooling). Instead of quantization of the RoI boundaries or bin bilinear interpolation is used to compute the exact values of the input features at four regularly sampled locations in each RoI bin, and aggregate the result (using max or average). **Backbone architecture** Faster RCNN uses a VGG like structure for extracting features from image, weights of which were shared among RPN and region detection layers. Herein, authors experiment with 2 backbone architectures - ResNet based VGG like in FRCNN and ResNet based [FPN](http://www.shortscience.org/paper?bibtexKey=journals/corr/LinDGHHB16) based. FPN uses convolution feature maps from previous layers and recombining them to produce pyramid of feature maps to be used for prediction instead of single-scale feature layer (final output of conv layer before connecting to fc layers was used in Faster RCNN) **Training Objective** The training objective looks like this ![](https://i.imgur.com/snUq73Q.png) Lmask is the addition from Faster RCNN. The method to calculate was mentioned above #### Observation Mask RCNN performs significantly better than COCO instance segmentation winners *without any bells and whiskers*. Detailed results are available in the paper |

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