Amol Kapoor

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A Two-Line History of ML

Written February, 2023.

I realized that a solid chunk of my self-ascribed predictive prowess comes from a pair of pretty straightforward observation about how ML has developed since the 90s:

As t → ∞:

  • we develop models that have less 'human' knowledge baked into the system;
  • because we have fewer priors, we (have to?) dramatically increase the amount of data the model is trained on.
A Just So Story

The plural of anecdote isn't data, but here are some interesting observations nonetheless.

Early ML models (pre 1980s) are essentially statistics. Everything from the data we collect to the models we use are decided by humans who have deep expertise in a given field, and use that expertise to make assumptions that make the task at hand computationally feasible. For example, the classic Eigenfaces paper. Quoting from the abstract:

Our approach treats face recognition as a two-dimensional recognition
problem, taking advantage of the fact that faces are normally upright
and thus may be described by a small set of 2-D characteristic views.

That's a lot of assumptions! I think in this era of ML, we (have to?) encode the problem in our solution in order to make any progress.

Neural networks, SVMs, and decision trees (1985-1995) are a step towards generality. Instead of having to choose between a range of models, hyperparameters, and data constraints, researchers have a catch-all model that is theoretically capable of learning any function. In this era, feature engineering is the name of the game — in order to get your ML models to play nice, you need to feed in very specific datasets. But your actual model is freed from having to encode the problem space. See Yann Lecun:

The usual method of recognizing individual patterns consists in
dividing the system into two main modules shown in figure 1. The first
module, called the feature extractor, transforms the input patterns so
that they can be represented by low dimensional vectors or short
strings of symbols that (a) can be easily matched or compared, and (b)
are relatively invariant with respect to transformations and
distortions of input patterns that do not change their nature. The
feature extractor contains most of the prior knowledge and is rather
specific to the task. It is also the focus of most of the design
effort, because it is often entirely hand-crafted. The classifier, on
the other hand, is often general-purpose and trainable. One of the
main problems with this approach is that recognition accuracy is
largely determined by the ability of the designer to come up with an
appropriate set of features. This turns out to be a daunting task
which, unfortunately, must be redone for each new problem. 

Right, well, feature engineering is all well and good but it also sucks. Not to worry, Yann has an answer. From the same paper:

The main message of this paper is that better pattern recognition
systems can be built by relying more on automatic learning and less on
hand-designed heuristics. This is made possible by recent progress in
machine learning and computer technology. Using character recognition
as a case study, we show that hand-crafted feature extraction can be
advantageously replaced by carefully designed learning machines that
operate directly on pixel images. 

He's talking, of course, of convolutional networks. Convolutional networks take all that pesky feature engineering out of the question. No more edge detectors, no more fourier transforms. Just feed the image in, and the model can do the rest.

This era of ML, roughly 20 years from 1998 to 2017, is all about encoding human priors into the model instead of into the data. In this time frame, a few core models appear for various modalities. ConvNets (and their descendents, like UNets) take advantage of geospatial relationships inherent to pixels in an image. RNNs, and later, LSTMs exploit natural patterns occurring in sequential data like language and audio. And representation learning algorithms like word2vec find success by encoding human-intuitions when learning semantics.

This is obviously a huge time period, and I'm glossing over a lot of stuff. During these 20 years, Big Data enters the lexicon, and then becomes standard practice. Mobile meaningfully comes online. GPUs start getting hacked by hobbyists for compute, then CUDA becomes a thing, then Theano, then Tensorflow. So over this timeframe, we are able to start really juicing our ML models with more data and compute. But even so, our approach is broadly the same, just at different scales. The same three models carried for a long time, and though people were trying to come up with new architectures nothing was really that promising or ever really stuck. (I'm consciously doing a disservice to GANs, GNNs, etc. etc. but that's why this is a just-so story!)

In 2017, an understated paper called Attention is all you Need appears on arxiv, which describes the Transformer. I doubt the authors of that paper knew what they were unleashing on the world; with hindsight, it's rather funny just how focused the authors were on text translation, when actually they had published the first meaningful change to how we approach ML in over two decades. Transformers go a step farther than ConvNets or RNNs — they don't make any assumptions about the underlying structure of the data, and as such are much more 'tabula rasa'. Because the transformer does not have pre-existing information about how to exploit patterns in data, you need a ton of data to train it; but because it is so flexible, it can eventually, with enough data, out-perform less generalized models.

And, in 2021, someone finally got around to collecting enough data. After scraping essentially the entire internet, OpenAI releases GPT3, showing how a system with essentially no baked-in assumptions and a ton of data beats out everything else.

And then in 2023 they went a step farther and built the worst comedian of all time.

Closing Thoughts

Is anything that I've said 100% correct? No, I don't think so. This post is more like evolutionary biology than genomics, reader be warned. But like I've said about ML in the past: all models are wrong, but some are useful.

What's the pattern in this model above? Increase generality. Increase data.

This actually dovetails pretty well with my time in research/ML publishing. There are, essentially, only three kinds of ML paper:

In other words, the long arc of ML is inherently towards more generalized models. Bit by bit, we're clawing our way towards that holy grail of generalized learning.


O, one last thing. I love speculating wildly about the future. So, what is the next big breakthrough in ML? Like as not, it's something we haven't seen before. But if my intuition is correct, and I had to pick from what we know in the field already, I think we should see an uptick in the usage and applications of message passing neural networks. These guys are strict generalizations of transformers, and so should be useful for a huge range of multimodal tasks.

The tricky thing about MPNNs is that we haven't really figured out how to scale them to the size of the big transformer stacks. And they require a lot more graph data (in keeping with the trend above). But multimodality and generality are big watchwords in the AI world these days, so I'm expecting someone to crack this nut sooner rather than later.