Paper Summary: Exploring the Limits of Transfer Learning with a Unified Text-to-text TransformerLast updated:
Please note This post is mainly intended for my personal use. It is not peer-reviewed work and should not be taken as such.
Authors create a framework to convert all text-based language problems1 into a purely text-to-text format, and use it to survey and compare several strategies to apply LLMs to all sorts of NLP tasks.
The resulting framework/model is called T5, short for Text-to-Text Transfer Transformer.
They analyze how the performance of each task gets better as we add more parameters, more data, change the pretraining objective and/or change the network architecture (but all are variations of the original Vaswani Transformer).
Authors have been inspired by other attempts at unifying NLP tasks2
They start off by pre-training a standard Encoder-Decoder Transformer with Self-attention as detailed on Section 2.1 on a masked LM task.
They then re-run the flow modifiying one the following elements:
1) Architecture: The basic Encoder-Decoder Architectire. Using just the decoder as a LM, using a single transformer. Using a prefix-LM instead of a regular LM. Also parameter sharing configuration.
2) Model capacity: Varying the number of layers, parameters and computation cycles
3) Objective function for pretraining: Vanilla language modelling trained on a chunk of text to predict the next chunk; the modified masked LM function. Varies corruption strategies, corruption rates and corrupted text length.
used by unsupervised pretraining.
Source: Raffel et al 2020.
4) Dataset type and size: Raw C4 baseline (unfiltered, unpreprocessed); preprocessed C4; News datasets; Reddit data; Wikipedia data, etc. Also truncated versions of datasets.
the C4 dataset for pre-training
yields better performance.
Source: Raffel et al 2020.
5) Fine-tuning Strategies: Adapter layers; Gradual Unfreezing.
6) Pretraining Strategies: Pre-training then Fine-tuning; Multi-tasking.
Text preprocessing is done with
SentencePiece(it splits text into a predetermined number of subword units)
The pretraining objective is a masked language modelling task (instead of predicting the next token as in regular LM, this masks a token anywhere in the sentence and the model is trained to find out what token it was)
"Concurrent work (Lan et al.,2019) also found that sharing parameters across Transformer blocks can be an effective means of lowering the total parameter count without sacrificing much performance."
"... we confirm the widely-held conception that using a denoising objective always results in better downstream task performance compared to a language modeling objective."
Pretraining with Fine-tuning resulted in better performance than just multi-tasking. But the best alternative is to mix in some multi-task objectives into pretraining and then doing fine-tuning.
The most efficient way of adding extra computing power seems to be to split it into more data and more training steps, equally, or else just investing it all into more data.
Using more data and higher-capacity for T5 (authors tested up to 11 billion parameters) resulted in higher performance in all tasks.
- However, just adding tons of extra capacity (1T+) to the baseline models did not make it surpass T5 with 4 billion parameters.
Single models (such as T5) are cheaper to use than similar-capacity ensembles of models.
The T5 model was not able to reach SOTA results in any Translation tasks
- But they did all pre-training on English-only datasets
- They claim that scale alone in an English-only dataset is not enough to surpass smaller models that use both languages in training sets.
T5 framework "... the text-to-text framework allows us to directly apply the same model, objective, training procedure, and decoding process to every task we consider"
"Note that we only pre-train on English data, so in order to learn to translate a given model will need to learn to generate text in a new language."
- This is quite interesting. The model learns to generate text in a language it has never seen during pre-training; just fine-tuned on
Scale "... scaling up may continue to be a promising way to achieve better performance. However, it will always be the case that there are applications and scenarios where using a smaller or less expensive model is helpful, for example when performing client-side inference or federated learning"
Relative embeddings: "Instead of using a fixed embedding for each position, relative position embeddings produce a different learned embedding according to the offset between the “key” and “query” being compared in the self-attention mechanism."
Model is built off Tensorflow
The name of the task itself is just another word prepended to the input text the model takes... amazing.
Authors say that preventing validation data from leaking into the training sets are a major issue when dealing with massive-scale datasets. Makes sense.
Model Capacity: 11 billion Parameters
Fill-in the blanks self-supervision is actually a generalization of vanilla (predict-the-next-word) pretraining used by other LLMs. (it's just a special case where the blank is at the end)
Section 2.1 (The Model) is a pretty good overview of what a modern Transformer architecture instantiation looks like.
Interesting take here, about the quality of CommonCrawl data: "Unfortunately, the majority of the resulting text is not natural language. Instead, it largely comprises gibberish or boiler-plate text like menus, error messages, or duplicate text."
- Here, they also say how they preprocessed/filtered the data to arrive at higher-quality text for training. Been there, done that
"Language Models as knowledge bases" is a thing, it's being studied. The task on which these are tested is apparently called "Closed-Book Q&A"
They cite an interesting paper that tried to represent every NLP task as span-extraction (just like the NLP Decathlon did it for Q&A)
1: Includes tasks such as: question answering, translation, document summarization, sentiment classification, document classification.
2: Including our previously summarized text Natural Language Decathlon which tries to cast all NLP tasks as instances of question-answering.