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Goodfellow, Bengio, Couville (2016) "Deep Learning", Chap. 7

Regularization for Deep Learning

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Goodfellow, Bengio, Couville (2016) "Deep Learning", Chap. 7

  1. 1. Regularization for Deep Learning Goodfellow, Bengio, & Courville (2016) Deep Learning, Chap 7. Shigeru ONO (Insight Factory) DL 読書会: 2020/08 Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 1 / 40
  2. 2. TOC 1 7.1 Parameter Norm Penalties 2 7.2 Norm Penalties as Constrained Optimization 3 7.3 Regularization and Under-Constrained Problems 4 7.4 Dataset Augmentation 5 7.5 Noise Robustness 6 7.6 Semi-Supervised Learning 7 7.7 Multitask Learning 8 7.8 Early Stopping 9 7.9 Parameter Tying and Parameter Sharing 10 7.10 Sparse Representation 11 7.11 Bagging and Other Ensemble Methods 12 7.12 Dropout 13 7.13 Adversarial Training 14 7.14 Tangent Distance, Tangent Prop and Manifold Tangent Classifier Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 2 / 40
  3. 3. (introduction) Regularization: any modification we make to a learning algorithm that is intended to reduce its generalization error possibly at the expense of increasing training error In the context of DL, most regularization strategies are based on regularizing estimators Possible situations (See Chap.5) : (1) the model family excluded the true DGP (underfitting) (2) the model family matched the true DGP (3) the model family included the true DGP but also many other possible DGP (overfitting) The goal of regularization is to take the model from (3) into (2). But... In most applications of DL, the true DGP is outside the model family (=(1)). Controlling the complexity of the model is not to find the model of the right size, but to find the model with appropriate regularization in which generalization error is minimized. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 3 / 40
  4. 4. 7.1 Parameter Norm Penalties Adding a parameter norm penalty Ω(θ) to the objective function J. ˜J(θ; X, y) = J(θ; X, y) + αΩ(θ) α(≥ 0): weight of the relative contribution of Ω. For NN, we typically choose Ω that penalizes only w (the weights of the affine transformation at each layer). It is reasonable to use the same α at all layers. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 4 / 40
  5. 5. 7.1.1 L2 Parameter Regularization Ω(θ) = 1 2 ||w||2 aka. weight decay, ridge regression, Tikhonov regularization. Bayesian interpretation: MAP inference with a Gaussian prior on the weights. (See 5.6.1) Total objective function: ˜J(w; X, y) = J(w; X, y) + α 2 w⊤ w Parameter gradient: ∇w ˜J(w; X, y) = αw + ∇wJ(w; X, y) What happens in a single gradient step? ... The learning rule is modified to shrink w by a constant factor. w ← (1 − ϵα)w − ϵ∇wJ(w; X, y) Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 5 / 40
  6. 6. 7.1.1 L2 Parameter Regularization What happens over the entire course of training? (in general) Unregularized: Let w∗ be the weights which minimize the unregularized objective function: w∗ = arg min w J(w). Make a quadratic approximation to the J(w) in the neighborhood of w∗ : ˆJ(θ) = J(w∗ ) + 1 2 (w − w∗ )⊤ H(w − w∗ ) where H is the Hessian matrix of J with respect to w evaluated at w∗ . The minimum of ˆJ occurs where its gradient ∇w ˆJ(w) = H(w − w∗ ) is 0. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 6 / 40
  7. 7. 7.1.1 L2 Parameter Regularization (Cont’d) Regularized: Let ˜w be the weights with minimize the regularized objective function ˜J. The minimum of ˜J occurs where α˜w + H(w − w∗ ) = 0. It follows that ˜w = (H + αI)−1 Hw∗ H is real and symmetric. We can have a eigenvalue decomposition H = QΛQ⊤ . ˜w = Q(Λ + αI)−1 ΛQ⊤ w∗ . i.e. The weight decay rescales w∗ along the axes defined by the eigenvector of H. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 7 / 40
  8. 8. 7.1.1 L2 Parameter Regularization What happens over the entire course of training? (in the case of linear regression) Unregularized: Cost function: (Xw − y)⊤ (Xw − y) Solution: w = (X⊤ X)−1 X⊤ y Regularized: Cost function: (Xw − y)⊤ (Xw − y) + 1 2 αw⊤ w Solution: w = (X⊤ X + αI)−1 X⊤ y. i.e. The regularization cause the algorithm to ”perceive” that X has higher variance (than the variance it really has). Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 8 / 40
  9. 9. 7.1.1 L2 Parameter Regularization Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 9 / 40
  10. 10. 7.1.2 L1 Regularization Ω(θ) = ||w||1 = ∑ i |wi| Total objective function : ˜J(w; X, y) = J(w; X, y) + α||w||1 Parameter gradient: ∇w ˜J(w; X, y) = αsign(w) + ∇wJ(w; X, y) It does not admit clean algebraic solution. For simple linear model with a quadratic cost function, ∇w ˆJ(w) = H(w − w∗ ) Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 10 / 40
  11. 11. 7.1.2 L1 Regularization (Cont’d) Assume the Hessian is diagonal, H = diag([H11, . . . , Hnn]) (i.e. no correlation between the input features) Then we have a quadratic approximation of the cost function: ˆJ(w; X, y) = J(w∗ ; X, y) + ∑ i ( 1 2 Hii(wi − w∗ i )2 + α|wi| ) The solution is: wi = sign(w∗ i ) max ( |w∗ i | − α Hii , 0 ) Consider the situation where w∗ i > 0 for all i. Then When w∗ i ≤ α Hii , the optimal value is wi = 0. When w∗ i > α Hii , the optimal value is just shifted by a distance α Hii . Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 11 / 40
  12. 12. 7.1.2 L1 Regularization (Cont’d) In short, the solution is more sparse (i.e. some parameter have an optimal value of zero). It has been used as a feature selection mechanism. E.g. LASSO Bayesian interpretation: MAP inference with a isotropic Laplace prior on the weights. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 12 / 40
  13. 13. 7.2 Norm Penalties as Constrained Optimization We can think of the penalties as constraints. Cost function: ˜J(θ; X, y) = J(θ; X, y) + αΩ(θ) If we wanted to constrain as Ω(θ) < k, we could construct a generalized Lagrangian L(θ, α; X, y) = J(θ; X, y) + α(Ω(θ) − k) The solution is θ∗ = arg min θ max α,α≥0 L(θ, α) we can fix α as its optimal value α∗ : θ∗ = arg min θ L(θ, α∗ ) = arg min θ J(θ; X, y) + α∗ Ω(θ) This is same as the problem of minimizing ˜J. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 13 / 40
  14. 14. 7.2 Norm Penalties as Constrained Optimization Sometimes we may with to use explicit constraints rather than penalties: when we know the appropriate value of k when the penalties can cause optimization to get stuck in local minima corresponding to small θ. when we with to impose some stability on the optimization procedure Approach: Srebro & Shraibman (2005): constraining the norm of each column of the weight matrix of a layer Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 14 / 40
  15. 15. 7.3 Regularization and Under-Constrained Problems Sometimes regularization is necessary for ML problems to be properly defined. when the problem depends on (X⊤ X)−1 but X⊤ X is singular. when the problem has no closed form solution. E.g., logistic regression applied to a problem where the class are linear separable. If weight w can achieve perfect classification, 2w will also achieve with higher likelihood. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 15 / 40
  16. 16. 7.4 Dataset Augmentation Idea: Create fake data and add it to the training set. an effective technique particularly for object recognition. E.g. translating the training images a few pixels in each direction. Injecting noise in the input to a NN. It can improve the robustness of NNs. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 16 / 40
  17. 17. 7.5 Noise Robustness Idea: Add noise to the weights. It can be interpreted as a stochastic implementation of Bayesian inference over the weight. Noise reflect our uncertainty on the model weights. It can also be interpreted as equivalent to a more traditional form of regularization. Consider we wish to train a function ˆy(x) using the least-square cost function J = Ep(x,y)[(ˆy(x) − y)2 ] Assume that we also include a random perturbation ϵw ∼ N(ϵ; 0, ηI) of the network weights. The objective function becomes ˜JW = Ep(x,y,ϵW)[(ˆyeW (x) − y)2 ] For small η, it is equivalent to J with a regularization term ηEp(x,y)[||∇Wˆy(x)||2 ]. It push the model into regions where the model is relatively insensitive to small variations in the weights, finding points that are not merely minima, but minima surrounded by flat regions. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 17 / 40
  18. 18. 7.5.1 Injecting Noise at the Output Targets Idea: Explicitly model the noise on the y labels. label smoothing: regularize a model based on a softmax with k output values by replacing classification target 0 with ϵ/(k − 1) 1 with 1 − ϵ Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 18 / 40
  19. 19. 7.6 Semi-Supervised Learning Idea: Use both unlabeled example (from P(x)) and labeled example (from P(x, y)) in order to estimate P(y|x) In the context of DL, semi-supervised learning usually refers to learning a representation h = f(x). The goal is to learn a representation so that examples from the same class have similar representations. Construct models in which a generative model of either P(x) or P(x, y) shares parameters with a discriminative model of P(y|x) One can find a better trade-off of two types of criterion: The supervised criterion: − log P(y|x) The unsupervised (generative) criterion: − log P(x) or − log P(x, y) Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 19 / 40
  20. 20. 7.7 Multitask Learning Idea: Pool the examples arising out of several tasks The model can be divided into two parts: Task-specific parameters Generic parameters, shared across the tasks It can improve generalization and generalization error bounds Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 20 / 40
  21. 21. 7.7 Multitask Learning Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 21 / 40
  22. 22. 7.8 Early Stopping Idea : Obtain a model with the parameters at the point in time with the lowest validation set error (rather than with the latest parameters in the training process) the most commonly use form of regularization in DL can be interpreted as a hyperparameter (the number of training steps) selection algorithm requires a validation set, which is not fed to the model One can perform extra training (where all training data is used) after initial learning (with early stopping). Two basic strategies: Initialize the model again and retrain on all the data (for the same number of steps as the first round) Keep the parameter and continue training (but now using all the data). It is not as well behaved. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 22 / 40
  23. 23. 7.8 Early Stopping How early stopping acts as regularizer: Restricting both the number of iterations and the learning rate limit the volume of parameter space reachable from the initial parameter value. In a simple linear model with a quadratic error function and simple gradient decent, early stopping is equivalent to L2 regularization. [...skipped...] Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 23 / 40
  24. 24. 7.8 Early Stopping Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 24 / 40
  25. 25. 7.9 Parameter Tying and Parameter Sharing Sometimes we may know there should be some dependencies between the parameters. Parameter Tying: E.g. two models performing the same classification task but with different input distributions: ˆy(A) = f(w(A)) , x), ˆy(B) = f(w(B)) , x) We believe the model parameters should be close to each other We can use a penalty Ω(w(A) , w(B) ) = ||w(A) − w(B) ||2 Parameter Sharing: force sets of parameters to be equal can lead to significant reduction of memory The most popular use: convolutional neural network (CNNs) (See Chap.9) Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 25 / 40
  26. 26. 7.10 Sparse Representation Idea: place a penalty on the activation of the unit (rather than on the parameters) Norm penalty regularization of representation: h: sparse representation of the data x add a norm penalty on the representation Ω(h) to the loss function J: ˜J(θ; X, y) = J(θ; X, y) + αΩ(h) We can use L1 penalty Ω(h) = ||h||1 or other types of penalties Orthogonal matching pursuit (OMP-k): encodes x with h that solves the constrained optimization problem arg min h,||h||0<k ||x − Wh||2 where ||h||0 is the number of nonzero entries of h OMP-1 can be a very effective feature extractor for DL Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 26 / 40
  27. 27. 7.11 Bagging and Other Ensemble Methods Ensemble methods combine several models (trained separately) in order to reduce generalization error an example of a general strategy called model averaging On average, the ensemble will perform at least as well as any of its members. If the members make independent errors, the ensemble will perform significantly better. Bagging (bootstrap aggregating) construct k different datasets of same size by sampling with replacement from the original dataset Model i is trained on data set i Boosting: construct an ensemble with higher capacity then individual models Boosting of NN: incrementally add NN to the ensemble Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 27 / 40
  28. 28. 7.12 Dropout Background: Bagging involves training multiple models and evaluating them on each test example. This seems impractical when each model is a large NN. Dropout can be thought of as a method of making bagging practical. What is Dropout? make all subnetworks that can be formed by removing nonoutput units from an base network In many cases, we can remove a unit by multiplying its output value by zero. Let µ be a vector of binary mask, which is applied to all the input and hidden units. train them with a minibatch-based algorithm Each time we load an example into a minibatch, we randomly sample µ and apply it. Typically, an input unit is included with probability 0.8, and a hidden unit is included with 0.5. Run forward propagation, back-propagation, and the learning update. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 28 / 40
  29. 29. 7.12 Dropout Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 29 / 40
  30. 30. 7.12 Dropout Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 30 / 40
  31. 31. 7.12 Dropout How to make a prediction: At training time, µ is sampled from the probability distribution p(µ) Each submodel defined by µ defines a probability distribution p(y|x, µ) To make a prediction from all submodels, we can use arithmetic mean:∑ µ p(µ)p(y|x, µ) But geometric mean performs better. Let ˜pensemble(y|x) be the geometric mean of p(y|x, µ). ˜p(y|x) is not guaranteed to be a probability distribution. We must renormalize: pensemble(y|x) = ˜pensemble(y|x) ∑ y′ ˜pensemble(y′|x) Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 31 / 40
  32. 32. 7.12 Dropout Weight scaling inference rule: We can approximate pensemble by evaluating p(y|x) in one model This model uses all units, but with the weights going out of unit i multiplied by the probability of including unit i if an inclusion probability of a unit is 1/2, the weight of the unit is multiplied by 1/2 at the end of training, or the states of the unit is multiplied by 2 during training There is not yet any theoretical argument for this rule, but empirically it performs well Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 32 / 40
  33. 33. 7.12 Dropout Advantages of dropout: very computationally cheap it does not significantly limit the type of model or training procedure Limitations: it reduces the effective capacity of a model. To offset this effect, we must increase the size of the model. it is less effective when extremely few labeled training examples are available. When additional unlabeled data is available, unsupervised feature learning can gain an advantage over dropout. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 33 / 40
  34. 34. 7.12 Dropout fast dropout: analytical approximations to the sum over all submodels more principled approach than the weight scaling inference rule. Interpretation of dropout: an experiments using ”dropout boosting” use exactly the same mask noise as dropout trains the entire ensemble to jointly (not independently) maximize the log-likelihood on the training set shows almost no regularization effect This demonstrates that dropout is a type of bagging. Dropout in itself have no robustness to noise. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 34 / 40
  35. 35. 7.12 Dropout Other approaches inspired by dropout: DropConnect: each product between a single scalar weight and a single hidden unit state is considered a unit that can be dropped Stochastic pooling: build ensembles of CNNs real valued mask: multiplying the weights by µ ∼ N(1, I) can outperform dropout Another view of dropout: Dropout regularize each hidden unit to be not merely a good feature but a feature that is good in many context. Masking can be seen as a form of highly intelligent, adaptive destruction of the information content of the input (rather than destruction of the raw input). It allows the model to make use of all the knowledge about the input distribution which has acquired so far. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 35 / 40
  36. 36. 7.13 Adversarial Training Adversarial example: an input x′ near a data point x such that the model output is very different at x′ In many cases, human observer cannot tell the difference between x and x′ One of the causes of these examples is excessive linearity in NN. The value of a linear function can change very rapidly if it has numerous inputs. Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 36 / 40
  37. 37. 7.13 Adversarial Training Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 37 / 40
  38. 38. 7.13 Adversarial Training Adversarial Training: training on adversarially perturbed examples from the training set a way of explicitly introducing a local constancy prior into NN Virtual adversarial example: Suppose the model assigns some label ˆy at a point x which has no true label. We can seek an adversarial example x′ that causes the model to output a label y′ (̸= ˆy) We can train the model to assign the same label to x and x′ This encourages the model to learn a function which is robust to small change This provide a means of semi-supervised learning Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 38 / 40
  39. 39. 7.14 Tangent Distance, Tangent Prop and Manifold Tangent Classifier Manifold hypothesis: the data lies near a low-dimensional manifold Tangent distance algorithm non-parametric nearest neighbor algorithm, where the distance between points x1 and x2 is the distance between the manifolds M1 and M2 to which they respectively belong approximate Mi by its tangent plane at xi The user has to specify the tangent vectors Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 39 / 40
  40. 40. 7.14 Tangent Distance, Tangent Prop and Manifold Tangent Classifier Tangent prop algorithm: [...skipped...] double backprop: [...skipped...] Shigeru ONO (Insight Factory) DL Chap.7 DL 読書会: 2020/08 40 / 40

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