This week, I am going to comment on a paper that appeared on arXiv on Tuesday and raised quite a lot of interest on Twitter. The title of the paper is Fixed Encoder Self-Attention Patterns in Transformer-Based Machine Translation and it describes work that has been done at the University of Helsinki.
Long story short: they removed almost everything why we though the Transformer architecture is so powerful, replaced it with simple rules and showed that nothing really changed. It seems to me that this paper nicely points out the methodological problems that the current natural language processing research has, but before I comment on that, let us have a look at what the paper really does.
The encoder of the Transformer architecture consists of two types of layers that alternate: self-attentive layers and feed-forward layers. The feed-forward layers do a non-linear transformation of the input which is nothing special, neural networks always did that. The self-attentive layers are what makes the Transformer so special and powerful. If we simplify that, we can say that every word uses several little search engines (= attention heads) via which it collects relevant pieces of information from other words (by computing a probabilistic distribution over them and taking the weighted average of projections of their representations). This is repeated on every layer of the network. Words share their little search engines (= attention heads) within layers, but use different ones on different layers. In the end, we have an information-rich representation of every input word that is further used in the decoder.
The Transformer was published in 2017 and pushed forward the state-of-the-art in machine translation by a large margin. Pretrained sentence representations based on the Transformer (BERT and his pre-trained friends) enabled similar progress in other natural language processing tasks one year later. When the first Transformer paper was published, it came out with optimistic claims about what the attention heads are capable of. The appendix of the paper showed what phenomena the attention heads learn to capture (without being explicitly taught to) such as dependency syntax or coreference chains. Afterward, several papers confirmed this optimism by showing what other interesting phenomena the self-attention heads learn to capture. No one forces the neural network to learn anything about the linguistics. It is trained to translate from one language into another and it by-the-way invents its own linguistic on the fly. This sounds really cool, but it is probably not important at all.
Another line of research focusing mostly on efficiency that showed that most of the attention heads can be pruned out with almost no harm to the performance of the models. The heads that survive this pruning are not those that do the fancy linguistic stuff, they mostly do quite trivial things.
This observation led the guys from Helsinki to a simple experiment. Instead of learning the attention heads in a standard way, they used 7 hardcoded attention heads:
one attending to the word itself,
two attending to neighboring words to the left and to the right,
two attending to a slightly larger left and right contexts,
one looking at the beginning of the sentence and one at the end,
and one attention head that is learned like in a standard Transformer. This kind of breaks the search engine metaphor. If no matter what you ask, the answer is the previous word, we can hardly call it searching. In the decoder, everything is learned as before.
The translation quality does not seem to be affected by this drastic simplification and it even improves for low-resource languages. It kind of suggests that a large proportion of the training data is used to learn that neighboring words matter the most. In an ablation study where they remove the heads one by one, they show that the head attending to the previous word is the most important one, more important than the trainable one.
And now I get to my concerns about what this story says about the current research in natural language processing. Google developed a monstrous black-box model that works stunningly well. Then I guess people were thinking something like this:
Because it works so well, it must know about complex linguistic phenomena the previous models were not able to grasp.
Because self-attention is the thing that is new about the model, the secret ingredient must be in the attention distributions.
At this moment, many researchers (including me) threw away their Occam’s razors and started to search for an empirical justification that it is really so. They found it, so it was possible to jump to a confusion: attention head allows capturing complex linguistic phenomena, this is why the Transformer works so well, not noticing this is circular reasoning. In the best tradition of western scientific skepticism, one would expect the opposite: nitpicking into all the details of the model, until we really find out why it works.
Meanwhile, the large black-box Transformer model finds use in industry and I wonder if this would be possible in other fields of engineering, for instance in car manufacturing. Imagine your car has a device that reduces fuel consumption. It is a pretty large complicated thing with many gears. It works well, but nobody really knows why. After two years, a serviceperson would tell you that they would disassemble half of the gears because people tried it and it worked just as well, still without knowing why exactly the device works. I can hardly imagine anyone would be happy to have something like this in a car. Yet, it does not seem to matter in search engines or while matching candidates’ CVs with job openings.
I am tempted to say one of the reasons this is tolerable in natural language processing is the overuse of the term artificial intelligence. There has always been something mysterious about intelligence. Developing an artificial one must be something more than pure engineering like car manufacturing. Maybe this subconscious feeling of a mystery is what makes people tolerate the opaqueness of the models.