S YMBOLIC L EARNING TO O PTIMIZE : T OWARDS I NTERPRETABILITY AND S CALABILITY
Recent studies on Learning to Optimize ( L2O ) suggest a promising path to automating and accelerating the optimization procedure for complicated tasks.
Existing
L2O models parameterize optimization rules by neural networks, and learn those numerical rules via meta-training.
However, they face two common pitfalls: (1) scalability : the numerical rules represented by neural networks create extra memory overhead for applying L2O models, and limit their applicability to optimizing larger tasks; (2) interpretability : it is unclear what an L2O model has learned in its black-box optimization rule, nor is it straightforward to compare different
L2O models in an explainable way.
To avoid both pitfalls, this paper proves the concept that we can “kill two birds by one stone”, by introducing the powerful tool of symbolic regression to L2O. In this paper, we establish a holistic symbolic representation and analysis framework for L2O, which yields a series of insights for learnable optimizers.
Leveraging our ﬁndings, we further propose a lightweight L2O model that can be meta-trained on large-scale problems and outperformed human-designed and tuned optimizers.
Our work is set to supply a brand-new perspective to L2O research.
Codes are available at: https: //github.com/VITA-Group/Symbolic-Learning-To-Optimize .
I NTRODUCTION
Efﬁcient and scalable optimization algorithms (a.k.a., optimizers ) are the cornerstone of almost all computational ﬁelds.
In many practical applications, we will repeatedly perform a certain type of optimization tasks over a speciﬁc distribution of data.
Each time, the inputs that deﬁne the optimization problem are new but similar to previous related tasks.
We say such an application has a narrow task distribution .
Conventional optimizers are theory-driven, so they obtain performance guarantees over the classes of problems speciﬁed by the theory.
Instead of manually designing and tuning task-speciﬁc optimizers, one may be naturally interested to pursue more general-purpose and data-driven optimization frameworks.
Learning to optimize ( L2O )
(Almeida et al., 2021; Andrychowicz et al., 2016; Chen et al., 2017;
Li & Malik, 2017a) is an emerging paradigm that automatically develops an optimizer by learning from its performance on a set of optimization tasks.
This data-driven procedure generates optimizers that efﬁciently solve problems similar to those in the training tasks.
Although often lacking a solid theoretical basis, the training process shapes the learned optimizer according to problems of interest.
When the task distribution is concentrated, the learned optimizer can speciﬁcally ﬁt the tasks by discovering “shortcuts” that classic optimizers do not take (Li & Malik, 2017a;b).
L2O has demonstrated many practical beneﬁts including faster convergence and/or better solution quality (Chen et al., 2021a), even saving clock time (Li et al., 2020b) and hardware energy
(Li et al., 2020a).
With promising progress being made, the questions persist: how far is L2O from becoming principled science?
To what extent can we trust L2O as being mature for real applications?
As a fast-growing new ﬁeld, fascinating open challenges remain concerning L2O, among which two particular barriers will be focused on in this work:
• Interpretability.
The learned optimizers are often hard to interpret .
Traditional algorithm design follows analytic principles and admits “white box” symbolic forms, which are clear
to explain, analyze, troubleshoot, and prove.
In contrast, existing L2O approaches ﬁt update models using sophisticated data-driven predictors such as neural networks.
As a result, it is unclear what rules have been learned in those “black boxes”, prohibiting their further explainable analysis and comparison.
• Scalability.
The learned optimizers can be hard to scale .
L2O methods use learnable predictors to ﬁt update rules, which incur extra overhead in memory that grows proportionally with the amount of optimization task variables.
Moreover, the training of a learned optimizer often involves backpropagating through a full unrolled sequence for the parameter updates in the optimization tasks, and the memory cost grows fast with the unroll length.
The scale of that growth results in complexities that current L2O methods cannot handle.
Our proposed solution, centered at addressing the two research gaps, is a brand-new symbolic representation and interpretation framework for L2O .
While beneﬁting from large modeling capacity, L2O update rules ﬁt by numerical predictors are inexplainable and memory-heavy.
In contrast, the symbolic representations of traditional optimizers enjoy low memory overhead and clear explainability.
We aim to integrate the strengths of symbolic forms into L2O.
We propose to convert a numerical L2O predictor into a symbolic form that preserves the same problem-speciﬁc performance and is still tunable.
Those symbolic rules are light, scalable, and interpretable.
They can even be made tunable and subject to further meta testing, by a lightweight trainable re-parameterization.
Based on them, we will further develop a set of L2O interpretability metrics.
Our main technical innovations are summarized below:
• We build the ﬁrst symbolic learning to optimize ( symbolic L2O ) framework that is generally applicable to existing L2O approaches.
That includes deﬁning the optimizer symbolic space and the ﬁtting approach to convert a numeric L2O to its symbolic counterpart.
• Under the symbolic L2O framework, we establish a uniﬁed scheme for L2O interpretability.
Our synergistic analysis in the symbolic domain uncovers a number of new insights that are otherwise difﬁcult to observe, including what “shortcuts” L2O methods essentially learn and how they differs across various methods.
• The lightweight symbolic representation also allows L2O to scale better, and we introduce a lightweight trainable re-parameterization to enable further ﬁne-tuning of symbolic rules.
For the ﬁrst time , we obtain a symbolic optimizer that can be meta-trained on ResNetlevel (23.5 million parameters)
optimizee within 30 GB GPU memory, and achieved the state-of-the-art (SOTA) performance when applied to training larger (ResNet-152, 58. million parameters) or very different deep models (MobileNet v2 (Sandler et al., 2018)) on various datasets, outperforming existing manually designed and tuned optimizers.
R ELATED W ORKS
L2O lies at the intersection of ML and optimization research.
It overlaps with meta-learning (Vilalta & Drissi, 2002; Hospedales et al., 2021), which contributes a signiﬁcant portion of L2O development (Andrychowicz et al., 2016; Li & Malik, 2017a).
L2O captures two main aspects of meta learning: rapid learning within each task, and transferable learning across many similar tasks.
Using data-driven predictors to ﬁt update rules, recent L2O approaches have shown success for a wide variety of problems, from convex optimization (Gregor & LeCun, 2010), nonconvex optimization (Andrychowicz et al., 2016) and minmax optimization (Shen et al., 2021), to black-box optimization (Chen et al., 2017) and combinatorial optimization (Khalil et al., 2017).
Application areas beneﬁting the most from L2O methods include signal processing and communication (Borgerding et al., 2017;
Balatsoukas-Stimming & Studer, 2019;
You et al., 2020), image processing (Zhang & Ghanem, 2018;
Corbineau et al., 2019;
Chen et al., 2020b), medical imaging (Liang et al., 2020; Yin et al., 2021) and computational biology (Cao et al., 2019; Chen et al., 2019).
The pioneering L2O work (Andrychowicz et al., 2016) leverages a long short-term memory (LSTM) as the coordinate-wise predictor, which is fed with the optimizee gradients and outputs the optimizee parameter updates.
Chen et al.
(2017) takes the optimizee’s objective value history as the input state of a reinforcement learning agent, which outputs the updates as actions.
To enhance L2O generalization, (Lv et al., 2017) proposes random scaling and convex function regularizers tricks.
(Wichrowska et al.,
2017) introduces a hierarchical RNN to capture the relationship across the optimizee parameters and trains it via meta-learning on the ensemble of small representative problems.
Training LSTM-based L2O models is however notoriously difﬁcult, that is rooted in the LSTM-style unrolling during L2O meta-training (Tallec & Ollivier, 2017;
Metz et al., 2018;
Pascanu et al., 2013;
Parmas et al., 2019).
A practical optimizer may need to take thousands or more of iterations.
However, naively unrolling LSTM to this full length is impractical for both memory cost and trainability by gradient propagation.
Most LSTM-based L2O methods (Andrychowicz et al., 2016; Chen et al., 2017;
Lv et al., 2017; Wichrowska et al., 2017; Metz et al., 2018; Cao et al., 2019) hence take advantage of truncating the unrolled optimization, yet at the price of the so-called “truncation bias” (Lv et al., 2017) that hampers their learned optimizer’s generalization to unseen tasks (Chen et al., 2020a).
Most L2O approaches are “black boxes” with neither theoretical backup nor clear explanation on what they learn.
A handful of efforts have been made on “demystifying” L2O, mainly about linking or reducing their behaviors to those of some analytic, better understood optimization algorithms.
Maheswaranathan et al.
(2020) analyzes and visualizes the learned optimizers, to discover that they have learned canonical mechanisms including momentum, gradient clipping, learning rate schedules, etc.
Another well-known success was on interpreting the learned sparse coding algorithm (Gregor & LeCun, 2010), as related to a speciﬁc matrix factorization of the dictionary’s Gram matrix (Moreau & Bruna, 2017), to the projected gradient descent trade-off between convergence speed and reconstruction accuracy (Giryes et al., 2018), or simply reduced to the original iterative algorithm with data-driven hyperparameter estimation (Chen et al., 2018; Liu et al., 2019; Chen et al., 2021b).
Other examples include analyzing the learned alternating minimization on graph recovery (Shrivastava et al., 2020), and an analogy between deep-unfolded gradient descent and Chebyshev step-sizes (Takabe &
Wadayama, 2020).
However, they are all restricted to some speciﬁc objective or algorithm type.
T ECHNICAL A PPROACH 3. N OTATION AND P RELIMINARIES
Without loss of generality, let us consider an optimization problem min x f ( x ) where x ∈ R d .
Here, f ( x ) is termed the optimization problem or optimizee , x is the variable to be optimized, and the algorithm to solve this problem is termed the optimizer .
A classic optimizer usually iteratively updates x based on a handcrafted rule.
For example, the ﬁrst-order gradient descent algorithm takes an update at iteration t based on the local gradient at the instantaneous point x t : x t +1 = x
t − α ∇ f ( x t ) , where α is the step size.
As a new learnable optimizer, L2O has much greater room to ﬁnd ﬂexible update rules.
We deﬁne the input of L2O as z t .
In general, z t contains the optimization historical information available at time t , such as the current/past iterates x 0 , . . .
, x t , and/or their gradients ∇ f ( x 0 ) , . .
. , ∇ f ( x t ) , etc.
L2O models an update rule by a predictor function g of z t : x t +1 = x
t − g ( z t , φ ) , where the mapping of g is parameterized by φ .
Finding an optimal update rule can be formulated mathematically as searching for a good φ over the parameter space of g .
Practically, g is often a neural network (NN).
Since NNs are universal approximators (Hornik et al., 1989), L2O has the potential to discover completely new update rules without relying on existing rules.
In order to ﬁnd a desired φ associated with a fast optimizer, (Andrychowicz et al., 2016) proposed to minimize the weighted sum of the objective function f ( x t ) over a time span T :
[Equation]
with x t +1 = x t − g
[Equation]
z t
[Equation]
t
[Equation] [Equation]
[Equation] [Equation]
T
[Equation]
where w 0 , . .
. , w T − 1 are the weights whose choices depend on empirical settings.
T is also called the unrolling length.
f represents a sample optimization task from an ensemble T that represent the target task distribution.
Note that φ determines the objective value through determining the iterates x t .
L2O solves the problem (1) for a desirable φ and correspondingly the update rule g ( z t , φ ) .
A typical L2O workﬂow is divided into two stages (Chen et al., 2021a): a meta-training stage that learns the optimizer with a set of similar optimization tasks from the task distribution; and a metatesting stage that applies the learned optimizer to new unseen optimization tasks.
The meta-training process often occurs ofﬂine and is time consuming.
However, the online application of the method at meta-testing is (aimed to be) time saving.
3. M OTIVATION AND C HALLENGES
If we take a uniﬁed mathematical perspective, the execution of an optimizer can be represented either through a symbolic rule, or by numeric computation.
Current L2O approaches parameterize their update rules g by numerical predictors that can be learned from data via meta-training.
They often choose sophisticated predictive models such as LTSMs (Andrychowicz et al., 2016;
Lv et al., 2017;
Chen et al., 2020a; Wichrowska et al., 2017), that predict the next update based on the current and historical optimization variables.
Despite the blessing of large learning capacity, the NN-based update rules are uninterpretable “black boxes”.
It is never well understood what rules existing L2O models have learned; nor is it easy to compare different L2O models and assess which has discovered more advanced rules.
Moreover, numerical predictors, especially RNN-based ones, limit L2O scalability through severe memory bottlenecks (Metz et al., 2019; Wu et al., 2018).
Their meta-training involves backpropagating through a full unrolled sequence of length T , for the parameter updates in their optimization task.
At meta-testing time (though requiring less memory than meta-training), one has to store RNN’s own parameters φ and hidden states, which are still far from negligible.
Those important gaps urge us to look back at symbolic update rules.
Almost all traditional optimizers can be seen as formula-based symbolic methods within our framework, such as stochastic gradient descent (SGD), Adam, RMSprop, and so on.
Their forms hinge on manual discovery and expert crafting.
A symbolic optimizer has little memory overhead, and can be easily interpreted or transferred like an equation or a computer program (Bello et al., 2017; Runarsson & Jonsson, 2000; Orchard & Wang, 2016; Bengio et al., 1994).
A handful of L2O methods tried to search for good symbolic rules from scratch, using evolutionary or reinforcement learning (Bello et al., 2017; Real et al., 2020;
Runarsson & Jonsson, 2000; Orchard & Wang, 2016; Bengio et al., 1994).
Unfortunately, those direct search methods quickly become inefﬁcient when the search space of symbols becomes large.
The open question remains as : how to strike the balance between the tractability of ﬁnding an update rule, and the effectiveness, memory efﬁciency and interpretability of the found rule?
In what follows, we will unify symbolic and numerical optimizers with a synergistic representation and analysis framework.
In brief, we stick to numerical predictors at meta-training for its modeling ﬂexibility and ease of optimization.
We then convert numerical predictors to symbolic rules, which preserve almost the same effectiveness yet have negligible memory overhead and clear explainability.
Facilitated by the new symbolic representations, we will develop a new suite of L2O interpretability metrics, and can meanwhile scale up L2O to much larger optimization tasks.
3. A S YMBOLIC D ISTILLATION F RAMEWORK FOR L2O
We will ﬁrst demonstrate how to convert the update predictor g : x t +1 = x
t − g ( z t , φ ) into symbolic forms that preserve the same effectiveness.
Contrasting with numerical forms, symbolic rules bear two advantages: (i) being white-box functions and enabling interpretability (see Section 3.3); and (ii) being much lighter , e.g., unlike RNNs that have to carry their own weights and hidden states, which removes the memory bottleneck for scaling up to large optimization tasks (see Section 3.4).
To achieve this goal, we leverage a classical tool of symbolic regression (SR) (Cranmer, 2020), a type of regression analysis that searches the space of mathematical expressions to ﬁnd an equation that best ﬁts a dataset, both in terms of accuracy and simplicity.
Different from conventional regression techniques that optimize the parameters for a pre-speciﬁed model structure, SR infers both model structures and parameters from data.
Popular algorithms rely on genetic programming to evolve from scratch (Runarsson & Jonsson, 2000; Gustafson et al., 2005; Orchard & Wang, 2016).
However,
SR algorithms are slow on large problems and rely on many heuristics, if evolving from scratch (Runarsson & Jonsson, 2000; Gustafson et al., 2005; Orchard & Wang, 2016).
They will become highly inefﬁcient if further being entangled with the rule search, like existing attempts demonstrated (Real et al., 2020; Bello et al., 2017; Runarsson & Jonsson, 2000; Orchard & Wang, 2016).
Inspired by the idea of “knowledge distillation” (Gou et al., 2021), we propose a symbolic distillation approach that decouples the rule search step (in the numeric domain) and the subsequent representation step (to the symbolic domain).
We will ﬁrst learn a numerical predictor g using existing
L2O methods (e.g., RNN-based), and then apply SR to ﬁt a symbolic form that approximates the input-output relationship captured by g .
In this way, the former step can stay with the more efﬁcient gradient-based search in a continuously parameterized space.
Speciﬁcally, we will generate an off-line database D of ( input, out ) pairs, then to train SR to minimize a ﬁtting loss on D .
Each pair is obtained from running the learned g on an optimization task: input is a sequence of z t generated during optimization within the ﬁxed unrolling length T ; and output is the corresponding sequence of the RNN teacher g ( z t , φ ) .
The training procedure of SR is to iteratively mutate a population of equation candidates, and ﬁlter out the better performing ones to come up with better equations.
The ﬁnal output of SR is a population of best equations composed of variables from input that best ﬁt output of its RNN teacher g
( z t , φ ) on the target task distribution, using operaters coming from the pre-deﬁned search space.
3. C ONCRETIZING THE S YMBOLIC D ISTILLATION F RAMEWORK
To make symbolic distillation for L2O executable, two open questions have to be deﬁned: (i) “what to ﬁt” , i.e., what symbolic operators we use as building blocks to compose the ﬁnal equation; and (ii) “ﬁt on what” , i.e., what information we consider as SR input operands.
These questions are open-ended and problem-dependent, and can be rather inconsistent among different L2O methods.
To make our approach more generalizable, we choose relatively straightforward options for both.
For the ﬁrst question , the selection of symbolic operators to use, we adopt the following set: { + , − , × , /, ( · ) 2 , √· , exp( · ) , x y , tanh , arcsinh , sinh , relu , erfc } .
The ﬁrst eight operators are adopted since they are the basic building blocks of several traditional optimizers such as Adam/- Momentum, and the rests are adopted since they are either the nonlinear activation functions used by typical L2O numerical predictors, or convey certain (update) thresholding effects which often expected and empirically used in existing deep learning optimization (Sun, 2019).
For a few operators whose input ranges are constrained to non-negative only, we extend them to be valid for any real-valued input (e.g., √ x is extended to sign( x ) ∗ (cid:112)
| x | , and x y is extended to sign( x ) ∗ | x | y , etc).
For the second question , we use the exact input variables of the numerical L2O
(e.g., coordinate wise gradients) as a default option throughout this paper.
To allow for further ﬂexibility, we also enable to expand the input operand set, to provide additional optimization-related features and let the
SR procedure decide which to use or not.
On a high level, we model the obtained symbolic equation as a Finite Impulse Response (FIR) ﬁlter, and set a maximum horizon T of the input variables.
In the following parts, unless otherwise speciﬁed, T is set to 20 (e.g., the latest 20 steps of input variables are visible to the rule).
To facilitate capturing inter-variable relationships, we re-scale all input variables so that all of them have the uniform scale variances of 1.
We conduct a proof-of-concept experiment to distill a learned rule using the LSTM-based L2O method (Andrychowicz et al., 2016).
The input sequence is z t = {∇ f ( x t , ) ∇ f ( x t − 1 ) , ..., ∇ f ( x t − T +1 ) } , i.e., the current and past gradients within an unrolling length T = 20.
The L2O meta-training was performed on multiple randomly-initialized LeNets on the MNIST dataset.
To ﬁt SR, we adopt a classical genetic programming-based SR approach (Koza, 1994).
The SR algorithm can score the equation complexity level as a function of the operator number, operator type diversity, and input variable number used (Cranmer et al., 2020).
Hence, we can apply a range of complexity levels, and obtain a group of SR ﬁtting rules from simple to complicated.
One example of our distilled symbolic rules is displayed as ( c i , i = 0,..., 19, are scalars, omitted for simplicity):
[Equation]
3. I NTERPRETABILITY FROM S YMBOLIC L2O R EPRESENTATIONS
For general ML models, the interpretability is referred to as the ability of the model to explain or to present in understandable terms to a human (Doshi-Velez & Kim, 2017).
When it speciﬁcally comes to an L2O model, its interpretability will be uniquely linked to the immense wealth of domain knowledge in optimization, from rigorous guarantees to empirical observations (Nocedal & Wright, 2006).
We hence deﬁne L2O interpretability as how well a learned optimizer’s behavior can align with the optimization domain knowledge, or be understandable to the optimization practitioners .
In fact, one may easily notice that the symbolic forms of L2O possess certain levels of “baked-in” interpretability.
For example, the example update rule (2) immediately supplies a few interesting observations: L2O discovers a momentum-like historic weight averaging mechanism, a time-decaying step size, as well as several normalization-like or (nonlinear) gradient clipping operations such as erfc , sinh and tanh .
All these are human-understandable as they represent well-accepted practices in deep learning optimization (Sun, 2019).
Our target is to establish a more principled and generally applicable L2O interpretability framework.
The new framework will be dedicated to capturing L2O characteristics, quantifying their behaviors, and meaningfully comparing different learned optimizers.
We propose two novel metrics that dissect
L2O along two dimensions: (i) Temporal Perception Field (TPF), i.e., how long an input sequence z t (past optimization trajectory)
the learned optimizer “effectively” takes advantage of; and (ii) Mapping
Complexity (MC), i.e., how complicated a predictor model g needs to “effectively” be.
The estimations of TPF and MC are both facilitated in the symbolic domain.
For TPF, take Eqn.
(2) for example again: the predicted update at time t involves a weighted (nonlinear) summation of past gradients ∇ f ( x t − i )) , i = 0 , · · · , T (set as 20).
c i can be seen as importance indicators for ∇ f ( x t − i ) , and a preliminary idea to deﬁne TPF is simply the weighted mean of historical lengths:
Temporal Perception
[Equation]
One challenge will be to estimate TPF when more complicated symbolic forms arise, e.g., the update rule has more heterogeneous compositions such as a i ( ∇ f ( x t − i ))
p i + b i ( ∇ f ( x t − i ))
q i + c i tanh ( ∇ f ( x t − i )) .
In such situations, more generic designs of TPF will have to be investigated and validated such as (cid:80) T − 1 i = | a i | + | b i | + | c i | (cid:80) T −i =
| a i | + | b i | + | c i | · i : we leave as future work.
For MC, the SR approach in (Cranmer et al., 2020) has deﬁned a complexity score for any resultant equation, that is calculated based on the total number and types of operators as well as input variables used for ﬁtting it.
Based on that tool, we set a threshold on the maximally allowable difference between the original numerical predictor and its symbolically distilled form (over some ( input, out ) validation set), and choose the minimum-complexity symbolic equation that falls under this threshold: its complexity score is used to indicate the intrinsic mapping complexity.
The above two deﬁnitions are in no way unique nor perfect: they are intended as our pilot study effort to quantify the understanding of L2O behaviors.
We will extend the above ideas to comprehensively examining the existing L2O approaches.
We aim at observing the relationship between the two metrics, and their empirical connections to L2O’s convergence speed, stability, as well as transferrability under task distribution shifts.
Extensive results will be reported in Section 4, from which we conclude a few hypotheses, including: (i) an L2O with larger TPF will converge faster and more stably on instances from the same task distribution, since it exploits more global optimization trajectory structure for this class of problems; and (ii) an L2O with smaller MC will be more transferable under distribution shifts, if meta-tuning (adaptation) is performed, since tuning a less complicated predictor model might be more data-efﬁcient.
We defer more details later in section 4.2 and Appendices A, B.
From the general taxonomy of interpretable deep learning (Li et al., 2021), our approach belongs to the category of using surrogate models , i.e., ﬁtting a simple and interpretable model (symbolic equation) from the original complicated one (numerical predictor), and using this surrogate model’s explanations to explain the original predictions.
Our new interpretability metrics and analyses will be the ﬁrst-of-its-kind for L2O.
The ﬁndings will provide troubleshooting tools for existing L2O methods, and insights for designing new L2O methods.
3. S CALABLE L2O T UNING WITH S YMBOLIC R EPRESENTATIONS
Symbolic L2O forms can not only be used as “frozen” update rules at testing, but also be tweaked to provide light-weight meta-tuning ability.
Taking Eqn.
(2) for example again, we can keep the equation form but set all c i to be trainable coefﬁcients, and continue to ﬁt them on new data, using differentiable meta-training or other hyperparameter optimization (HPO) methods (Feurer & Hutter, 2019).
This “tunability” is useful for adapting a learned optimizer to a shifted task distribution, such as to larger optimization tasks since the optimizer overhead now is much lighter compared to the
[Figure]
original numerical predictor.
The overall procedure of training numerical L2O
→ distill symbolic rule → meta tuning on new task are illustrated in Fig.
I NTERPRETABILITY A ND S CALABILITY E VALUATIONS F OR S YMBOLIC L2O
In this section, we systematically discuss our experimental settings and ﬁndings.
Section 4.1 veriﬁes symbolic regression’s ability to recover underlying optimization rules through sanity checks.
Section 4.2 leverages the interpretability of symbolic L2O to quantify several properties of the numerical
L2O models.
The scalability evaluation of the proposed symbolic L2O models is given in section 4.3.
4. S ANITY C HECKS F OR S YMBOLIC R EGRESSION O N O PTIMIZATION T ASKS
As discussed in section 3.3, with the help of a few adaptative approaches (e.g., operator selection and modiﬁcation, injection of inductive bias with processed features, variable magnitude re-scaling, etc.), the symbolic regression is capable to recover the underlying rules of the optimizers.
To further verify the reliability of the recovery, we conduct sanity checks by regressing known optimizers, whose results are presented in table 1.
The gradients are acquired from the training of ResNet-50 on Cifar-10, and the most recent 20 steps are kept for the symbolic regression inputs.
The hyperparameters for the symbolic regressions are tuned to our best efforts: details could be found in Appendix C. As demonstrated in table 1, the symbolic regression faithfully reproduces the known equations.
[Figure]
4. I NTERPRETABILITY OF T RADITIONAL O PTIMIZERS AND L EARNED O PTIMIZERS
In this section
[Figure]
we quantify the interpretability of the learned optimizers via two metrics TPF and MC which was deﬁned in section 3.5.
Without loss of generality
[Figure]
we consider three different problems (optimization tasks) and six representative numerical L2O backbones for the evaluation, described in
Tables 2 and 3.
The three problems are:
[Equation]
P 1 is a non-convex generalized Rastrigin function used in Cao et al.
(2019), where A ∈ R 10 × 10 and x ∈ R 10 .
Note that the elements in A , b , c are i.i.d.
and sampled from N (0 , 1) for simplicity.
For
P 2 / P 3 , we acquire the data and labels from the MNIST dataset, and f denotes a MLP with Relu activation.
The x indicats the parameters of the MLP and the size of the neurons for each layer is (50 , 20) for P 2 and (50 , 20 , 20 , 12) for P 3 , respectively.
On the other hand, the six numerical L2O backbones considered are: DM (Andrychowicz et al., 2016),
RP (Lv et al., 2017), DM+CL+IL (DM with imitation learning and curriculum learning proposed in (Chen et al., 2020a)), RP+CL+IL, RP (small) (architecture same as RP, but with fewer coefﬁcients: the dimension of its input projection is reduced from 20 to 6, and the number of layers of the LSTM is reduced from 2 to 1), and RP (small)(extra) (architecture same as RP (small) , but with extra augmenting
Adam-type gradient features into the input feature set).
Empirical observations.
We report the values of TPF and MC of the learned L2O models in table 3, and two observations could be made.
First, the larger TPF, the faster convergence speed is expected (DM+CL+IL is faster than DM, and RP+CL+IL is faster than RP).
Second, with more diverse features, the learned models performs better even with lower MC (RP (small)(extra) achieves the best performance among the variants of RPs and DMs, and its input features are the most diverse).
[Table] [Table]
4. S CALABILITY E VALUATIONS
In this section
[Table]
we evaluate the scalability of symbolic L2O on large-scale optimizees.
Due to the memory burden of LSTM-based L2O models, meta-training on large-scale problems are challenging for LSTM-based L2O models (Chen et al., 2020a; Bello et al., 2017)
[Table]
and most L2O models that work well with small-scale optimizees fails to scale up to large-scale optimizees, such as ResNet- (Chen et al., 2020a; Bello et al., 2017; Andrychowicz et al., 2016;
Lv et al., 2017).
Thanks to the simple form and the hidden-states-free property, symbolic L2O models are able to implement fast inference with little memory overhead during the training, which brings potentials to scale up to these larger-scale tasks.
In the following, we show the feasibility that a symbolic L2O model could achieve better performance than handcrafted tuned optimizers by meta-tuning it on large-scale optimizees.
In our experiments, the best performing numerical L2O model, i.e., RP (small)(extra) , is selected as the teacher to perform symbolic distillation.
We ﬁrst meta-pre-train a RP (small)(extra) model on P 1 to get a better initialization.
Then, we distill it into a symbolic equation followed by meta-ﬁne-tuning the distilled symbolic equation on the large-scale optimizees.
Meta-ﬁne-tuning for distilled symbolic equations.
Recall that RP (small)(extra) has three types of input features.
The ﬁrst two features are the same as the RP model’s
(Lv et al., 2017):
[Equation]
where g t denotes the gradients, m t
= [ β 1 m t − 1 + (1 − β 1 ) g t ] / (1 − β t 1 ) , and v t =
[ β 2 v t − 1 + (1 − β 2 )
g 2 t ] / (1 − β t 2 ) .
The third type of input features ˆ n t contains a few trainable parameters, i.e., k 1 , k 2 , l 1 , l 2 , α 1 , and α 2 .
The form of ˆ n t is similar to ˆ m t ’s except that g t in m t is replaced with k 1 g 1+ α 1 t + k 2 g 1 − α 1 t , and g 2 t in v t is replaced with l 1 g 2+ α 2 t +
[Equation]
As discussed in table 3, the
RP (small)(extra) model has the lowest MC and TPF values, and its performance is empirically veriﬁed by the output of symbolic distillation.
We summarize the the output symbolic form as follows:
[Equation]
where G = { ˆ m t , ˆ g t , ˆ n t } is the set of input features, W ∈ R 3 × L and γ is the trainable coefﬁcients.
Note that the coefﬁcient γ is only tuned during the meta-ﬁne-tuning stage.
The accuracy of the distilled equation 6 is veriﬁed since the R 2 score between the distilled equation and the original
RP (small)(extra) model is large enough (R 2 = 0 . 88 ).
The performance of the distilled equation is also reported in table 3.
Additionally, the plotted input-output curve can be referred in Appendix B).
For the evaluation, we pick three large-scale optimizees, i.e., ResNet-50, ResNet-152 and MobileNetV2, on the Cifar-10 and Cifar-100 datasets.
Due to the scale of the optimizees, it poses difﬁculty for the previous L2O models to execute training on a single GPU.
Thanks to the simple form and the hidden-states-free property, the meta-training process for our symbolic L2O only takes less than 30GB GPU memory over ResNet-50.
In addition, thanks to the initialization from pre-trained
RP (small) (extra) , it only takes one pass for the 200-epochs’ meta training before the symbolic L2O is ready.
[Table]
[Figure]
The evaluation results are reported in
Fig.2 and Table 4.
For the comparison, we include the following baselines: SGD optimizers with learning rate selected from (0.1, 0.01, 0.001), with momentum selected from (0.5, 0.9, 0.99), with and without the Nesterov momentum, with and without cosine learning rate annealing, and the
Adam optimizers with learning rate selected from (0.01, 0.001).
In the legend of Fig. 2, default values are: learning rate = 0.1, momentum = 0.9, without using Nesterov momentum and without using cosine learning rate annealing.
The setting of different hyperparameter selection are plotted in the legend of Fig.
Note that the LSTM-based L2O requires signiﬁcantly higher memory cost for large-scale optimizees, thus we do not include them in this section.
As we observed in Fig.2, the proposed symbolic L2O model outperforms most of the humanengineered optimizers (SGD, Adam, Nesterov, cosine learning rate decay, etc.) in three tasks, in terms of not only convergence speed but also accuracy.
[Figure]
C ONCLUSION
Current L2O methods face important limitations in terms of their interpretability and scalability.
In this work, we attempt to overcome those limitations, by unify symbolic and numerical optimizers with a synergistic representation and analysis framework.
We ﬁrst numerically learn a neural network based L2O model, then distill it into a symbolic equation to link the wealth of optimization domain knowledge; the distilled equation can be meta-tuned further on testing problems.
Symbolic L2O may hopefully reveal a new path to enhancing L2O to reaching the state-of-the-art practical performance.
We hope our results will boost the wider adoption of L2O methods, that can eventually replace the laborious manual design or case-by-case tuning of optimizers.
A M ORE D ETAILS R EGARDING T HE I NPTERPRETABILITY A. E XPERIMENTAL D ETAILS
As discussed in section 4.
we consider 6 models in the interpretability experiments: the DM, RP, and modiﬁcations of RP.
The RP model differ from the DM model in two ways: RP use two gradient features (discussed in equation where DM used the pure gradient as its input.
Additionally, RP has an input projection layer where DM does not.
A. M ORE D ETAILED D ISCUSSIONS
The message conveyed by the RP (small)(extra) distillation result.
Among the comparison of table the winner model is the RP (small)(extra) , which has the smallest evaluation loss.
At the same time, RP (small)(extra) is also the simplest model.
This does not mean that the optimal gradient descent rule is simplest rule, but rather, the optimal gradient descent rule is the simple combination of complex gradient features, as the input feature of RP (small)(extra) is the most complex one compared with other model, such as RP or
DM.
On the intuition behind TPF and MC: should they be large or small?
In table 3, the TPF (temporal perception ﬁeld) and MC (mapping complexity) are compared together with the performance (measured in the evaluation loss) of the model.
The TPF/MC are measured for certain models w.r.t.
its input, hence they are blind to the concrete components of the input.
We link the different behaviors of different models and their TPF and MC, with the following summaries:
• Comparing among models with the same type of input (DM vs. DM + CL + IL, or RP vs. RP + CL + IL), the larger TPF/MC lead to better evaluation loss on the testing set (newly sampled, unseen problems).
• Comparing across model with different input features (i.e., DM vs. RP, or RM + CL + IL vs. RP + CL + IL, or RP vs RP + RP (small)(extra) ), the model with more complex input feature set (e.g., RP) not only outperform the model with simpler input set (e.g., DM), but they also have smaller temporal perception ﬁeld and mapping complexity.
Hence, improving the input feature diversity for the numerical L2O model will reduce both TPF and MC, while still leading to better performance
The inﬂuence of pre-training problem selection on the ﬁnal performance of symbolic L2O. Since obtaining the symbolic L2O model require a meta-pre-training step, one straightforward question may arise: what if the meta-pre-training step selected a different problem, how will the ﬁnal results be affectd?
To test the sensitiveness of the symbolic L2O to its meta-pre-train initialization step, we conducted ablation studies for the experiment in section 4.3 as follows.
We set up 3 different problem settings as the meta-pre-training problems, and pre-trained the RP (small)(extra) (best performing) and the DM (worst performing) models.
After that, we applied the symbolic distillation procedure, read out the distilled equation skeleton, set its coefﬁcients as trainable, then meta-ﬁne-tune the symbolic rule in the ResNet-50 on Cifar10.
The three different problem we chose are: P 1 , 2 introduced in section section 4.2, and P 4 , which is also minimizing
We distill symbolic rule from the optimization trajectory data of the RP (small)(extra) and DM models on three different problems.
For RP (small)(extra) , the distilled symbolic equations take the same skeleton, which is exactly equal to equation (6), just with different coefﬁcients.
However, with the DM model, the symbolic skeletons are signiﬁcantly different, as shown as follows:
[Equation]
Note that the most desired output from the symbolic regression step is the equation skeleton, and the coefﬁcients are subject to be further ﬁne-tuned in the meta-ﬁne-tune stage.
Hence, the message from the ablation study results is that:
• When the meta-pre-train problem are changed, the knowledge learned by the L2O model with more diverse input features (RP (small)(extra) ) could be more aligned under the same frame, compared with the L2O model with fewer input features (DM).
In other words, more diverse input feature diversity of the optimizer will bring better tolerance/stability to the meta-pre-train problem selection.
• When the equation’s skeleton is distilled
correctly, the choice of the meta-pre-train problem does not have signiﬁcant effect on the resulting symbolic L2O’s performance.
The interpretations of the distilled equations 6 and 7.
The equation 6 is the linear combination of multiple thresholded gradient features.
First, these diverse features (gradient, Adam, learned exponential combination) enable higher chance to escape from ﬂat local minima.
Second, the nonlinear thresholding potentially smoothes noisy gradient signal and made the update more stable.
Equations 7 also have interpretable patterns.
In P 1 , the asinh is a thresholding function which clip the gradient, and inside it is log( x + 1) with x being the big square root.
The function y = log( x + 1) is close to the function y
= x when the magnitude of x is small, where the gradient magnitude is indeed usually small.
In P 2 , the equation is close to SGD in the ﬁrst term − 0 .
02 g t .
If the sign of g t and g t − 1 is the same, i.e., two most recent gradient holds the same sign, then sign( g t g t − 1 ) =
1 , it descent more, otherwise it descent less.
In P 4 , the equation is similar to Adam with additional tanh thresholding to certain components.
How much adaptation is needed during meta-ﬁne-tune.
This experiment follows the same setting of the above adaptation experiments for DM and RP (small)(extra) : meta-pre-train on P 1 / P 2 / P 4 , and metaﬁne-tune and evaluate on the ResNet-50 on Cifar10.
To help understand how much improvement is needed in ﬁne-tuning, the evaluation performances (measured in test set accuracies) before and after the ﬁne-tuning step are provided in table 5.
From the table, it can be seen that the symbolic rule from RP (small)(extra) , which have smaller MC, transfers better than DM.
High complexity model DM, on the other hand, are less stable for different meta-pretraining problem.
[Table]
Why could the symbolic distillation provide better interpretability.
With regard to our claim in section 3.3: white-box functions have better interpretability and lighter weight , one question may arise: the interpretability and light weight properties do not seem unique to symbolic optimizers, but rather, they seem unique to simple optimizers.
Indeed, the simpler model will naturally hold better interpretability than the complex model.
However, whether the rule learned by the numerical optimizer is simple or not is not revealed by the numerical optimizer itself: it is only after the proposed symbolic distillation is applied can people realize that the rule learned by RP is simple (has small MC).
It is also only after the symbolic distillation uncovered this fact can people safely simplify the RP model into the RP (small) model, to make it lighter weight and obtain better interpretability and better performance (see table 3).
In other words, the interpretability indeed belongs to simple optimizers, but a numerical optimizer can not tell the simplicity of the rules it has learned, prior to applying the proposed symbolic distillation.
In this sense, the proposed symbolic distillation brings better interpretability to numerical L2O.
The running time of the proposed method.
The proposed symbolic distillation procedure contains the following components: the meta-pre-training, SR step, meta-ﬁne-tuning, and the ﬁnal evaluation to optimize the large scale problem.
The execution time for each step is provided in table 6.
The proposed TPF and MC metrics are all simple calculations based on the SR results, hence they are instantly available once the SR results are out.
The execution times for SR and computing TPF/MC are measured on the 2.6 GHz Intel Core i7 CPU with 16 GB 2400 MHz DDR4 Memory, and other other computation times are measured on the Nvidia A6000 GPU.
[Table]
B M ORE D ETAILS R EGARDING T HE S CALABILITY B. E XPERIMENTAL D ETAILS
During the symbolic regression step, the outputs are a list of distilled equation candidates.
For each candidate, a unique complexity value could be measured using Cranmer (2020).
During the sanity check in section 4.1, the complexity levels are chosen to be the largest output, which are relatively small ( < 5 for SGD, < 50 for Momentum, 100 for Adam).
During the experiments in section 4.3, we pick the equation with 100 complexity value, since this number gives the best ﬁtting ability as well as the best performance (check ﬁgure 5 and table 8 for details) according to our extensive experiments.
We have also tested varying the random seeds for both meta-pre-train and the symbolic regression, and check the resulting symbolic form.
Due to the low MC of the learned rule of RP (small)(extra) , the distilled equation stably shows the same form (equation 6), regardless of the random seeds.
We used 128 batch size, and in both meta-ﬁne-tune phase and ﬁnal evaluation phase, we trained the
CNN optimizees for 200 epochs.
B. M ORE D ETAILED D ISCUSSIONS
Veriﬁcation of the symbolic rule ﬁtting ability.
We veriﬁed that the distilled equation and the original optimizer are well aligned.
First, ﬁgure 3 shows the ﬁtting curve of the symbolic equation against the numerical model.
In this ﬁgure, we chose a ﬁxed time during the optimization, and plot the output of both original numerical L2O and the distilled symbolic equation.
The results show that the distilled equation ﬁts the original model well.
Second, in table 7, we have distilled different optimizers, and have shown SR’s ﬁtting ability for each, indicated by R 2 -score.
The high R 2 -score further veriﬁed the ﬁtting ability is reliable for our tasks.
The interpretations of the symbolic L2O ultimately used in section 4.3.
In section 4.3, we used the best numerical model (RP (small)(extra) ) as the benchmark to distill the symbolic equation, and the distilled equations for RP (small)(extra) in most cases take the form of equation (6).
We note that this simple nonlinear thresholding function yielded good ﬁtting accuracy, with the R 2 -score 0.88.
We note that the simple
[Table]
[Figure]
form has already been veriﬁed in the small mapping complexity and temporal perception ﬁeld of
RL si in table 3.
Performance comparison of the numerical and distilled symbolic rules.
In table 3, we offered the performance comparison between the numerical rules and their distilled symbolic surrogates.
To further illustrate their convergence speed, we plot the optimization trajectories for the DM model (the worst performing one) and the RP (small)(extra) model (the best performing one) in ﬁgure 4.
Speciﬁcally, the lines in ﬁgure 4 correspond to the following rows/columns in table 3: the Evaluation Loss by numerical L2O and the Evaluation Loss by distilled equation , for the DM and RP (small)(extra) models.
[Figure]
The relationship between SR complexity and ﬁtting ability/accuracy.
The symbolic regression algorithm is able to generate a series of equations with different complexity.
Intuitively, the oversimple equations under ﬁts the L2O model, and the over-complex equations tends to overﬁt.
The exact ﬁtting curve is shown in Fig.
In this ﬁgure, the y coordinate is the R 2 -score of the ﬁtting performance, the higher the better, and the x -coordinate is the complexity of the equation.
With the increase of the complexity, the ﬁtting performance tends to ﬁrst increase then decrease.
The inﬂuence of complexity to the accuracy are displayed in table 8.
The experimental setting in this table is the same setting as section 4.3, and the numerical optimizer used is also RP (small)(extra) .
[Figure]
[Table]
Interpreting the hyperbolic functions presented in the distilled equations.
The hyperbolic functions appears in the distilled equations.
There are two possible reasons for the occurances of the hyperbolic functions.
First, the symbolic equations are learned through gradient-free random mutation, hence it is possible that such hyperbolic thresholding effect has a higher chance to reduce the uncertainty under noisy gradient data, which makes these operators to be more easily selected and kept among other mutated results.
Note that for tanh()/asinh()/etc, these functions are approximately equal to y = x function when the input magnitude is small (the case of tanh is plotted in ﬁgure 6).
Therefore, only when the inputs have large magnitude will adding these functions result in signiﬁcant difference in the output.
[Figure]
Second, the tanh() functions in the distilled equations of LSTM models are straightforward to understand: the symbolic rules are the surrogate of the LSTM model, and the LSTM uses the tanh() function as its non-linear activation function.
Therefore, the tanh() function in the distilled equations could be correctly reﬂecting the nonlinearity of the LSTM teacher’s updating dynamics.
We are more interested in the distillation of a real numerical L2O model, hence we have conducted a new series of experiments, by comparing not including and including the hyperbolic functions.
The distilled equations without hyperbolic functions take similar forms than with the hyperbolic functions,
[Table]
and are slightly different in the coefﬁcients.
The R 2 -score does not improve by dropping these functions.
This could be explained by that even given the hyperbolic functions, the equations with and without these functions are all compared and screened during the SR step, hence the resulting equation is already the best among them, though with the hyperbolic.
[Table]
Large scale evaluations on GLUE tasks
We have evaluated the symbolic rule used in section 4.3 on BERT with GLUE tasks.
We compared between symbolicL2O and the AdamW. For both optimizers, we used lr = 0 . 0005 .
For AdamW, we used β 1 = 0 .
9 , β 2 = 0 .
In all these experiments, we ﬁne-tuned the BERT (BERT base cased model in English) for 100 epochs.
For symbolicL2O, we ﬁxed the coefﬁcients, without meta-tuning them while ﬁne-tuning BERT.
The results are as follows.
From the results, the learned symbolicL2O achieved best results across all tasks.
C S YMBOLIC REGRESSION PRELIMINARIES
The psudo-code for the SR algorithm.
As described in section 1, the symbolic regression works by constantly mutating a list of equations to gradually obtain the better performing one.
The core mutation algorithm of SR is provided in ﬁgure 7 as a pseudo-code.
The hyperparameters that we chose.
There are several hyperparameters for the symbolic regression procedure that could potentially inﬂuence the performance.
We list them as follows:
• The number of iterations: it means the number of executions, among which the best equations are picked.
We set it to a large enough value, 300, since it shows in the experiment that any larger values leads to the similarly good results.
• The population number: it means the number of symbolic equation candidates used at each iteration.
It is empirically observed that simpler models is enough to be accurately distilled with smaller population number.
After tuning this parameter across all L2O models, we used the largest best population number, 200, for all models.
• Dataset size: it means the number of samples in the SR.
We used 5000 samples across the experiments, which is veriﬁed to be sufﬁcient for our purpose.
Algotithm: mutation operations in Symbolic Regression
function Random_Mutate (dataset): # cweights is the weights used to randomize mutation type.
# If equation too big, don't add new operators if n >= maxsize || depth >= maxdepth
cweights[3]
= 0.
cweights[4]
= 0.
end successful_mutation
= false mutationChoice = rand() # randomly select one mutation type.
# ------ Mutations ------ while (!successful_mutation) && attempts < max_attempts tree = copyNode(prev) successful_mutation
= true
if mutationChoice < cweights[1] tree = mutateConstant(tree, temperature, options) elseif mutationChoice < cweights[2]
# Can always mutate to the same operator tree = mutateOperator(tree, options) elseif mutationChoice < cweights[3]
# Can potentially have a situation without success tree =
appendRandomOp(tree, options, nfeatures) elseif mutationChoice < cweights[4] tree = insertRandomOp(tree, options, nfeatures) elseif mutationChoice < cweights[5] tree = deleteRandomOp(tree, options, nfeatures) elseif mutationChoice < cweights[6] tree = simplifyTree(tree, options)
# Sometimes we simplify tree tree = combineOperators(tree, options) return PopMember(tree, beforeLoss, parent=parent_ref) elseif mutationChoice < cweights[7] tree =
genRandomTree(5, options, nfeatures) else # no mutation applied return PopMember(tree, beforeLoss, parent=parent_ref) end attempts +=end if !
successful_mutation return PopMember(copyNode(prev), beforeLoss, parent=parent_ref) end if probChange < rand() return PopMember(copyNode(prev), beforeLoss, parent=parent_ref) else return PopMember(tree, afterLoss, parent=parent_ref) end