Posit AI Weblog: Infinite shock

Amongst deep studying practitioners, Kullback-Leibler divergence (KL divergence) is probably greatest recognized for its function in coaching variational autoencoders (VAEs). To study an informative latent house, we don’t simply optimize for good reconstruction. Quite, we additionally impose a previous on the latent distribution, and intention to maintain them shut – usually, by minimizing KL divergence.

On this function, KL divergence acts like a watchdog; it’s a constraining, regularizing issue, and if anthropomorphized, would appear stern and extreme. If we go away it at that, nonetheless, we’ve seen only one facet of its character, and are lacking out on its complement, an image of playfulness, journey, and curiosity. On this submit, we’ll check out that different facet.

Whereas being impressed by a collection of tweets by Simon de Deo, enumerating purposes of KL divergence in an enormous variety of disciplines,

we don’t aspire to supply a complete write-up right here – as talked about within the preliminary tweet, the subject might simply fill a complete semester of research.

The far more modest objectives of this submit, then, are

1. to rapidly recap the function of KL divergence in coaching VAEs, and point out similar-in-character purposes;
2. as an example that extra playful, adventurous “different facet” of its character; and
3. in a not-so-entertaining, however – hopefully – helpful method, differentiate KL divergence from associated ideas akin to cross entropy, mutual info, or free vitality.

Earlier than although, we begin with a definition and a few terminology.

KL divergence in a nutshell

KL divergence is the anticipated worth of the logarithmic distinction in possibilities in keeping with two distributions, (p) and (q). Right here it’s in its discrete-probabilities variant:

[begin{equation}
D_{KL}(p||q) = sumlimits_{x} p(x) log(frac{p(x)}{q(x)})
tag{1}
end{equation}]

Notably, it’s uneven; that’s, (D_{KL}(p||q)) is just not the identical as (D_{KL}(q||p)). (Which is why it’s a divergence, not a distance.) This side will play an essential function in part 2 devoted to the “different facet.”

To emphasize this asymmetry, KL divergence is usually known as relative info (as in “info of (p) relative to (q)”), or info achieve. We agree with certainly one of our sources that due to its universality and significance, KL divergence would in all probability have deserved a extra informative identify; akin to, exactly, info achieve. (Which is much less ambiguous pronunciation-wise, as effectively.)

KL divergence, “villain”

In lots of machine studying algorithms, KL divergence seems within the context of variational inference. Typically, for real looking knowledge, actual computation of the posterior distribution is infeasible. Thus, some type of approximation is required. In variational inference, the true posterior (p^*) is approximated by an easier distribution, (q), from some tractable household.
To make sure we’ve got a superb approximation, we decrease – in concept, at the very least – the KL divergence of (q) relative to (p^*), thus changing inference by optimization.

In apply, once more for causes of intractability, the KL divergence minimized is that of (q) relative to an unnormalized distribution (widetilde{p})

[begin{equation}
J(q) = D_{KL}(q||widetilde{p})
tag{2}
end{equation}]

the place (widetilde{p}) is the joint distribution of parameters and knowledge:

[begin{equation}
widetilde{p}(mathbf{x}) = p(mathbf{x}, mathcal{D}) = p^*(mathbf{x}) p(mathcal{D})
tag{3}
end{equation}]

and (p^*) is the true posterior:

[begin{equation}
p^*(mathbf{x}) = p(mathbf{x}|mathcal{D})
tag{4}
end{equation}]

Equal to that formulation (eq. (2)) – for a derivation see (Murphy 2012) – is that this, which reveals the optimization goal to be an higher sure on the damaging log-likelihood (NLL):

[begin{equation}
J(q) = D_{KL}(q||p^*) – log p(D)
tag{5}
end{equation}]

Yet one more formulation – once more, see (Murphy 2012) for particulars – is the one we really use when coaching (e.g.) VAEs. This one corresponds to the anticipated NLL plus the KL divergence between the approximation (q) and the imposed prior (p):

[begin{equation}
J(q) = D_{KL}(q||p) – E_q[- log p(mathcal{D}|mathbf{x})]
tag{6}
finish{equation}]

Negated, this formulation can also be known as the ELBO, for proof decrease sure. Within the VAE submit cited above, the ELBO was written

[begin{equation}
ELBO = E[log p(x|z)] – KL(q(z)||p(z))
tag{7}
finish{equation}]

with (z) denoting the latent variables ((q(z)) being the approximation, (p(z)) the prior, usually a multivariate regular).

Past VAEs

Generalizing this “conservative” motion sample of KL divergence past VAEs, we are able to say that it expresses the standard of approximations. An essential space the place approximation takes place is (lossy) compression. KL divergence offers a solution to quantify how a lot info is misplaced once we compress knowledge.

Summing up, in these and comparable purposes, KL divergence is “dangerous” – though we don’t need it to be zero (or else, why hassle utilizing the algorithm?), we definitely wish to hold it low. So now, let’s see the opposite facet.

KL divergence, good man

In a second class of purposes, KL divergence is just not one thing to be minimized. In these domains, KL divergence is indicative of shock, disagreement, exploratory conduct, or studying: This actually is the angle of info achieve.

Shock

One area the place shock, not info per se, governs conduct is notion. For instance, eyetracking research (e.g., (Itti and Baldi 2005)) confirmed that shock, as measured by KL divergence, was a greater predictor of visible consideration than info, measured by entropy. Whereas these research appear to have popularized the expression “Bayesian shock,” this compound is – I feel – not probably the most informative one, as neither half provides a lot info to the opposite. In Bayesian updating, the magnitude of the distinction between prior and posterior displays the diploma of shock caused by the info – shock is an integral a part of the idea.

Thus, with KL divergence linked to shock, and shock rooted within the basic means of Bayesian updating, a course of that may very well be used to explain the course of life itself, KL divergence itself turns into basic. We might get tempted to see it in all places. Accordingly, it has been utilized in many fields to quantify unidirectional divergence.

For instance, (Zanardo 2017) have utilized it in buying and selling, measuring how a lot an individual disagrees with the market perception. Greater disagreement then corresponds to larger anticipated positive factors from betting towards the market.

Nearer to the realm of deep studying, it’s utilized in intrinsically motivated reinforcement studying (e.g., (Solar, Gomez, and Schmidhuber 2011)), the place an optimum coverage ought to maximize the long-term info achieve. That is attainable as a result of like entropy, KL divergence is additive.

Though its asymmetry is related whether or not you employ KL divergence for regularization (part 1) or shock (this part), it turns into particularly evident when used for studying and shock.

Asymmetry in motion

Trying once more on the KL system

[begin{equation}
D_{KL}(p||q) = sumlimits_{x} p(x) log(frac{p(x)}{q(x)})
tag{1}
end{equation}]

the roles of (p) and (q) are essentially totally different. For one, the expectation is computed over the primary distribution ((p) in (1)). This side is essential as a result of the “order” (the respective roles) of (p) and (q) could should be chosen in keeping with tractability (which distribution can we common over).

Secondly, the fraction contained in the (log) signifies that if (q) is ever zero at a degree the place (p) isn’t, the KL divergence will “blow up.” What this implies for distribution estimation normally is properly detailed in Murphy (2012). Within the context of shock, it signifies that if I study one thing I used to suppose had likelihood zero, I will likely be “infinitely shocked.”

To keep away from infinite shock, we are able to be sure that our prior likelihood is rarely zero. However even then, the attention-grabbing factor is that how a lot info we achieve in anyone occasion is determined by how a lot info I had earlier than. Let’s see a easy instance.

Assume that in my present understanding of the world, black swans in all probability don’t exist, however they may … perhaps 1 p.c of them is black. Put in another way, my prior perception of a swan, ought to I encounter one, being black is (q = 0.01).

Now actually I do encounter one, and it’s black.
The knowledge I’ve gained is:

[begin{equation}
l(p,q) = 0 * log(frac{0}{0.99}) + 1 * log(frac{1}{0.01}) = 6.6 bits
tag{8}
end{equation}]

Conversely, suppose I’d been far more undecided earlier than; say I’d have thought the chances had been 50:50.
On seeing a black swan, I get quite a bit much less info:

[begin{equation}
l(p,q) = 0 * log(frac{0}{0.5}) + 1 * log(frac{1}{0.5}) = 1 bit
tag{9}
end{equation}]

This view of KL divergence, when it comes to shock and studying, is inspiring – it may lead one to seeing it in motion in all places. Nonetheless, we nonetheless have the third and ultimate process to deal with: rapidly evaluate KL divergence to different ideas within the space.

Entropy

All of it begins with entropy, or uncertainty, or info, as formulated by Claude Shannon.
Entropy is the typical log likelihood of a distribution:

[begin{equation}
H(X) = – sumlimits_{x=1}^n p(x_i) log(p(x_i))
tag{10}
end{equation}]

As properly described in (DeDeo 2016), this formulation was chosen to fulfill 4 standards, certainly one of which is what we generally image as its “essence,” and certainly one of which is particularly attention-grabbing.

As to the previous, if there are (n) attainable states, entropy is maximal when all states are equiprobable. E.g., for a coin flip uncertainty is highest when coin bias is 0.5.

The latter has to do with coarse-graining, a change in “decision” of the state house. Say we’ve got 16 attainable states, however we don’t actually care at that degree of element. We do care about 3 particular person states, however all the remaining are mainly the identical to us. Then entropy decomposes additively; whole (fine-grained) entropy is the entropy of the coarse-grained house, plus the entropy of the “lumped-together” group, weighted by their possibilities.

Subjectively, entropy displays our uncertainty whether or not an occasion will occur. Curiously although, it exists within the bodily world as effectively: For instance, when ice melts, it turns into extra unsure the place particular person particles are. As reported by (DeDeo 2016), the variety of bits launched when one gram of ice melts is about 100 billion terabytes!

As fascinating as it’s, info per se could, in lots of instances, not be one of the best technique of characterizing human conduct. Going again to the eyetracking instance, it’s fully intuitive that individuals have a look at shocking elements of photos, not at white noise areas, that are the utmost you can get when it comes to entropy.

As a deep studying practitioner, you’ve in all probability been ready for the purpose at which we’d point out cross entropy – probably the most generally used loss perform in categorization.

Cross entropy

The cross entropy between distributions (p) and (q) is the entropy of (p) plus the KL divergence of (p) relative to (q). If you happen to’ve ever carried out your personal classification community, you in all probability acknowledge the sum on the very proper:

[begin{equation}
H(p,q) = H(p) + D_{KL}(p||q) = – sum p log(q)
tag{11}
end{equation}]

In info theory-speak, (H(p,q)) is the anticipated message size per datum when (q) is assumed however (p) is true.
Nearer to the world of machine studying, for mounted (p), minimizing cross entropy is equal to minimizing KL divergence.

Mutual info

One other extraordinarily essential amount, utilized in many contexts and purposes, is mutual info. Once more citing DeDeo, “you may consider it as probably the most common type of correlation coefficient you can measure.”

With two variables (X) and (Y), we are able to ask: How a lot can we study (X) once we study a person (y), (Y=y)? Averaged over all (y), that is the conditional entropy:

[begin{equation}
H(X|Y) = – sumlimits_{i} P(y_i) log(H(X|y_i))
tag{12}
end{equation}]

Now mutual info is entropy minus conditional entropy:

[begin{equation}
I(X, Y) = H(X) – H(X|Y) = H(Y) – H(Y|X)
tag{13}
end{equation}]

This amount – as required for a measure representing one thing like correlation – is symmetric: If two variables (X) and (Y) are associated, the quantity of data (X) offers you about (Y) is the same as that (Y) offers you about (X).

KL divergence is a part of a household of divergences, known as f-divergences, used to measure directed distinction between likelihood distributions. Let’s additionally rapidly look one other information-theoretic measure that not like these, is a distance.

Jensen-Shannon distance

In math, a distance, or metric, moreover being non-negative has to fulfill two different standards: It have to be symmetric, and it should obey the triangle inequality.

Each standards are met by the Jensen-Shannon distance. With (m) a combination distribution:

[begin{equation}
m_i = frac{1}{2}(p_i + q_i)
tag{14}
end{equation}]

the Jensen-Shannon distance is a mean of KL divergences, certainly one of (m) relative to (p), the opposite of (m) relative to (q):

[begin{equation}
JSD = frac{1}{2}(KL(m||p) + KL(m||q))
tag{15}
end{equation}]

This might be a super candidate to make use of had been we involved in (undirected) distance between, not directed shock brought on by, distributions.

Lastly, let’s wrap up with a final time period, limiting ourselves to a fast glimpse at one thing entire books may very well be written about.

(Variational) Free Vitality

Studying papers on variational inference, you’re fairly more likely to hear individuals speaking not “simply” about KL divergence and/or the ELBO (which as quickly as you already know what it stands for, is simply what it’s), but in addition, one thing mysteriously known as free vitality (or: variational free vitality, in that context).

For sensible functions, it suffices to know that variational free vitality is damaging the ELBO, that’s, corresponds to equation (2). However for these , there may be free vitality as a central idea in thermodynamics.

On this submit, we’re primarily involved in how ideas are associated to KL divergence, and for this, we comply with the characterization John Baez offers in his aforementioned discuss.

Free vitality, that’s, vitality in helpful kind, is the anticipated vitality minus temperature occasions entropy:

[begin{equation}
F = [E] -T H
tag{16}
finish{equation}]

Then, the additional free vitality of a system (Q) – in comparison with a system in equilibrium (P) – is proportional to their KL divergence, that’s, the data of (Q) relative to (P):

[begin{equation}
F(Q) – F(P) = k T KL(q||p)
tag{17}
end{equation}]

Talking of free vitality, there’s additionally the – not uncontroversial – free vitality precept posited in neuroscience.. However sooner or later, we’ve got to cease, and we do it right here.

Conclusion

Wrapping up, this submit has tried to do three issues: Having in thoughts a reader with background primarily in deep studying, begin with the “ordinary” use in coaching variational autoencoders; then present the – in all probability much less acquainted – “different facet”; and at last, present a synopsis of associated phrases and their purposes.

If you happen to’re involved in digging deeper into the numerous varied purposes, in a spread of various fields, no higher place to begin than from the Twitter thread, talked about above, that gave rise to this submit. Thanks for studying!