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Title: Bayesian Inference for Large Scale Image Classification

Paper content:
Bayesian inference promises to ground and improve the performance of deep neural networks.
It promises to be robust to overfitting, to simplify the training procedure and the space of hyperparameters, and to provide a calibrated measure of uncertainty that can enhance decision making, agent exploration and prediction fairness.
Markov Chain Monte Carlo (MCMC) methods enable Bayesian inference by generating samples from the posterior distribution over model parameters.
Despite the theoretical advantages of Bayesian inference and the similarity between MCMC and optimization methods, the performance of sampling methods has so far lagged behind  optimization methods for large scale deep learning tasks.
We aim to fill this gap and introduce ATMC, an adaptive noise MCMC algorithm that estimates and is able to sample from the posterior of a neural network.
ATMC dynamically adjusts the amount of momentum and noise applied to each parameter update in order to compensate for the use of stochastic gradients.
We use a ResNet architecture without batch normalization to test ATMC on the Cifar10 benchmark and the large scale ImageNet benchmark and show that, despite the  absence of batch normalization, ATMC outperforms a strong optimization baseline in terms of both classification accuracy and test log-likelihood.
We show that ATMC is intrinsically robust to overfitting on the training data and that ATMC provides a better calibrated measure of uncertainty compared to the optimization baseline.
In contrast to optimization approaches in machine learning that derive a single estimate for the weights of a neural network, Bayesian inference aims at deriving a posterior distribution over the weights of the network.
This makes it possible to sample model instances from the distribution over the weights and offers unique advantages.
Multiple model instances can be aggregated to obtain robust uncertainty estimates over the network's predictions; uncertainty estimates are crucial in domains such as medical diagnosis and autonomous driving where following a model's incorrect predictions can result in catastrophe (Kendall & Gal, 2017) .
Sampling a distribution, as opposed to optimizing a loss, is less prone to overfitting and more training doesn't decrease test performance.
Bayesian inference can also be applied to differential privacy, where each individual sample has increased privacy guarantees (Wang et al., 2015) , and to reinforcement learning, where one can leverage model uncertainty to balance between exploration and exploitation (Osband & Van Roy, 2017) .
Traditional Markov Chain Monte Carlo (MCMC) methods like HMC (Neal et al., 2011) are a standard class of methods for generating samples from the posterior distribution over model parameters.
These methods are seldom applied in deep learning because they have traditionally failed to scale well with large datasets and many parameters (Rajaratnam & Sparks, 2015) .
Stochastic Gradient MCMC (SG-MCMC) methods have fared somewhat better in scaling to large datasets due to their close relationship to stochastic optimization methods.
For example the SGLD sampler (Welling & Teh, 2011) amounts to performing stochastic gradient descent while adding Gaussian noise to each parameter update.
Despite these improvements, samplers like SGLD are only guaranteed to converge to the correct G t ← minibatch gradient(θ t ) 6:
distribution when the step size is annealed to zero; additional control variates have been developed to mitigate this to some extent (Ahn et al., 2012; Ding et al., 2014) .
The objective of this work is to make Bayesian inference practical for deep learning by making SG-MCMC methods scale to large models and datasets.
The contributions described in this work fall in three categories.
We first propose the Adaptive Thermostat Monte Carlo (ATMC) sampler that offers improved convergence and stability.
ATMC dynamically adjusts the amount of momentum and noise applied to each model parameter.
Secondly, we improve an existing second order numerical integration method that is needed for the ATMC sampler.
Third, since ATMC, like other SG-MCMC samplers, is not directly compatible with stochastic regularization methods such as batch normalization (BatchNorm) and Dropout (see Sect. 4), we construct the ResNet++ network by taking the original ResNet architecture (He et al., 2016) , removing BatchNorm and introducing SELUs (Klambauer et al., 2017) , Fixup initialization (Zhang et al., 2019a) and weight normalization (Salimans & Kingma, 2016) .
We design ResNet++ so that its parameters are easy to sample from and the gradients are well-behaved even in the absence of BatchNorm.
We show that the ATMC sampler is able to outperform optimization methods in terms of accuracy, log-likelihood and uncertainty calibration in the following settings.
First, when using the ResNet++ architecture for both the ATMC sampler and the optimization baseline, the ATMC sampler significantly outperforms the optimization baseline on both Cifar-10 and ImageNet.
Secondly, when using the standard ResNet for the optimization baseline and the ResNet++ for the ATMC sampler, multiple samples of the ATMC that approximate the predictive posterior of the model are still able to outperform the optimization baseline on ImageNet.
Using the ResNet++ architecture, the ATMC sampler reduces the need for hyper-parameter tuning since it does not require early stopping, does not use stochastic regularization, is not prone to over-fitting on the training data and avoids a carefully tuned learning rate decay schedule.
The empirical results show it is possible to sample the posterior distribution of neural networks on large scale image classification problems like ImageNet.
A major obstacle for sampling the posterior of ResNets in particular is the lack of compatibility with BatchNorm.
Using recent advances in initialization and the SELU activation function we are able to stabilize and speed up training of ResNets without resorting to BatchNorm.
Nonetheless, we observe that BatchNorm still offers a unique advantage in terms of generalization per-formance.
We hope that future work will allow the implicit inductive bias that BatchNorm has to be transferred into an explicit prior that is compatible with sampling methods.
Multiple posterior samples provide a much more accurate estimate of the posterior predictive, and consequently much better accuracy and uncertainty estimates.
For inference, making predictions using a large ensemble of models sampled from the posterior can be costly.
Variational Inference methods can be used to quickly characterize a local mode of the posterior (Blundell et al., 2015) .
More recent work shows that a running estimate of the mean and variance of the parameters during training can also be used to approximate a mode of the posterior (Maddox et al., 2019) .
Methods like distillation could potentially be used to compress a high-quality ensemble into a single network with a limited computational budget (Balan et al., 2015) .
Although the form in (4) is very general, alternative methods for dealing with stochastic gradients have been proposed in the literature.
One approach is to estimate the covariance of the stochastic gradient noise B explicitly and use it correct and pre-condition the sampling dynamics (Ahn et al., 2012; Li et al., 2016) .
Other sampling methods are not based on an SDE that converges to the target distribution.
Under some conditions stochastic optimization methods can be interpreted as such a biased sampling method (Mandt et al., 2017) .
Predictions based on multiple samples from the trajectory of SGD have been used successfully for obtaining uncertainty estimates in large scale Deep Learning (Maddox et al., 2019) .
However, these methods rely on tuning hyperparameters in such a way that just the right amount of noise is inserted.

assistant: text: We scale Bayesian Inference to ImageNet classification and achieve competitive results accuracy and uncertainty calibration.