Homework 2: Prefix Tuning for Text Generation

Start on June 12, 2023 | Due on June 26, 2023

Getting Started

If you have already cloned my homework repository nlp-class-hw for previous homeworks then go into that directory and update the directory:

git pull origin/master
cd nlp-class-hw/prefixtune

If you don’t have that directory anymore then simply clone the repository again:

git clone https://github.com/anoopsarkar/nlp-class-hw.git

Clone your own repository from GitLab if you haven’t done it already:

git clone git@csil-git1.cs.surrey.sfu.ca:USER/advnlpclass-1234-g-GROUP.git

Note that the USER above is the SFU username of the person in your group that set up the GitLab repository.

Then copy over the contents of the prefixtune directory into your hw2 directory in your repository.

Set up the virtual environment:

python3.10 -m venv venv
source venv/bin/activate
pip3 install -r requirements.txt

You must use Python 3.10 (or later) for this homework.

Note that if you do not change the requirements then after you have set up the virtual environment venv you can simply run the following command to get started with your development for the homework:

source venv/bin/activate

Background

The goal for this homework is to learn to use an autoregressive language model which processes the input from left to right and predicts the next token based on the left context (also called a causal language model when there is a distinct input and output but is trained and decoded as if it was a left to right LM).

We will be using a text to table task, which takes structured data in the form of a table and produces a text description of what is in the data.

For example, if the input is:

The table is linearized and stored as follows:

name : Alimentum | area : city centre | family friendly : no

This linearized table is given as input to a language model to produce a text description of the table:

There is a place in the city centre , Alimentum , that is not family - friendly .

However, there can be many ways to produce a text description. Here are some of the ways you can produced a text for the same table:

There is a place in the city centre , Alimentum , that is not family - friendly .
In the city centre there is a venue name Alimentum , this is not a family - friendly venue .
Alimentum is not a family - friendly place , located in city centre .
Alimentum is not a family - friendly arena and is located in the city centre .
Alimentum is not a family - friendly place in the city centre .
Alimentum in city centre is not a family - friendly place .

A large language model is trained on data from many diverse sources and there might be a mix of structured and unstructured data in the training data helping it to solve this task if you prompt the language model in the right way.

The goal of this assignment is to fine-tune a language model (one that isn’t exposed to the entire web) in a compute-efficient way using prefix tuning. The end result is that the parameters for the large language model are not even modified after fine-tuning and we only learn an appropriate (continuous) prompt that can solve this task using in-context learning.

Data set

The data set is the E2E table to text task as described in detail in the following publication:

The E2E Dataset: New Challenges For End-to-End Generation. Jekaterina Novikova, Ondřej Dušek, Verena Rieser. SIGDIAL 2017 short paper.

The leaderboard for this task shows that there is still room for improvement on the state-of-the-art.

Data files

The data files provided are:

• data/train.txt.gz – the training data used to train the answer/default.py model (default.py uses the huggingface dataset for E2E to load the data but a copy of the training data is provided just in case).
• data/input – input files dev.txt and test.txt infected with noise. a subset of dev.txt is provided as small.txt for development of your solution.
• data/reference/dev.out – the reference output for the dev.txt input file
• data/reference/small.out – the reference output for the dev.txt input file

Default solution

The default solution is provided in answer/default.py. To use the default as your solution:

cd answer
cp default.py prefixtune.py
cp default.ipynb prefixtune.ipynb
cd ..
python3 zipout.py
python3 check.py

The default solution will use the language model directly using a prompt to solve the task. default.py has additional code provided that will look for a fine-tuned model as data/peft.pt and use that for decoding if it exists. Otherwise, there is code in default.py that loads up the dataset for fine-tuning. You will need to copy and modify default.py to add your own fine-tuning steps to train a better model for this task.

Please do not commit your model file or the language model file into your git repository as it is moderately large and you can go over your disk quota.

To run the default program as follows:

python3 answer/default.py -i data/input/small.txt > output.txt

And then you can check the score on the dev output file called output.txt by running:

python3 bleu.py -t data/reference/small.out -o output.txt

which produces the evaluation as a BLEU score:

bleu score: 1.3688755959761818

For this homework we will be scoring your solution based on the BLEU score which is described in detail in the Accuracy section below. However the BLEU score is not the only focus. You can focus on efficiency, model size, experimental comparison with other approaches and many other choices.

Make sure that the command line options are kept as they are in answer/default.py. You can add to them but you must not delete any command line options that exist in answer/default.py.

Submitting the default solution without modification will get you zero marks.

Accuracy

The accuracy for this homework is calculated using the BLEU score which compares n-gram overlap between the candidate and the reference. It then combines that with a brevity penalty to make sure the candidate is approximately matching the length of the reference.

• Reference: The NASA Opportunity rover is battling a massive dust storm on Mars .
• Candidate: A NASA rover is fighting a massive storm on Mars .

The candidate has 11 tokens (so 11 unigrams, 10 bigrams, 9 trigrams and 8 fourgrams). The following n-gram matches can be found:

• Unigram: A, NASA, rover, is, a, massive, storm, on, Mars Score = 9/11
• Bigram: rover is, a massive, storm on, on Mars, Mars . Score = 5/10
• Trigram: storm on Mars, on Mars . Score = 2/9
• Fourgram: storm on Mars . Score = 1/8

In the BLEU score we actually use the minimum frequency of each n-gram based on reference versus candidate, e.g. if an n-gram occurs twice in the candidate, but only once in the reference then we take the numerator to be one.

The overall n-gram precision score is a equally weighted geometric mean of each of the n-gram precision values.

• Overall precision: (9/11 * 5/10 * 2/9 * 1/8)^(1/4) = 0.32649

The brevity penalty is calculated as the minimum of 1 and exp(1 - reference-length/output-length). For the above example, the reference length is 13 and the candidate length is 11 so the brevity penalty is 0.8337.

The BLEU score is the product of the overall precision score and the brevity penalty. For the above example, the BLEU score would be 0.27221.

Pytorch

You will need to use some Pytorch API calls and possibly the Huggingface Transformers package API to solve this homework. The following links will help you get started on what you need to know to get started. You can learn a lot of the Pytorch basics by understanding default.py.

Some useful links if you feel lost at the beginning:

• 60 mins intro to Pytorch
• Introduction to the transformers library

Read the source code in default.py in detail.

Prefix Tuning

You will implement the approach presented in this paper to solve the table to text fine-tuning challenge:

Prefix-Tuning: Optimizing Continuous Prompts for Generation. Xiang Lisa Li, Percy Liang. ACL 2021.

Hyperparameters

default.py uses distilgpt2 as the language model which is the smallest of the GPT2 models. You can alternatively use gpt2 but you should not use gpt2-large or gpt2-xl or any of the massively large language models since they can be very slow even on a GPU.

The other hyperparameters for prefix tuning are available by running:

python3 answer/default.py -h

You can experiment with different values for the number of virtual tokens and the prefix projection boolean flag. Although five virtual tokens seems to work the best when compared to the increased training time for more virtual tokens.

Setting the prefix projection boolean to True will improve results but at the cost of more parameters (0.1 percent of original model will go up to 10 percent).

You can experiment with different parameters to the model.generate() function in predict() or even replace it altogether. Explore useful parameters for the generate function.

Do not change any of the other hyperparameters unless you post on the discussion board and it is approved by the instructor.

Required files

You must create the following files:

• answer/prefixtune.py – this is your solution to the homework. start by copying default.py as explained below.
• answer/prefixtune.ipynb – this is the iPython notebook that will be your write-up for the homework.

Run your solution on the data files

To create the output.zip file for upload to Coursys do:

python3 zipout.py

For more options:

python3 zipout.py -h

Check your accuracy

To check your accuracy on the dev set:

python3 check.py

The output score is the BLEU score.

python3 check.py -h

In particular use the log file to check your output evaluation:

python3 check.py -l log

The accuracy on data/input/test.txt will not be shown. We will evaluate your output on the test input after the submission deadline.

Submit your homework on Coursys

Once you are done with your homework submit all the relevant materials to Coursys for evaluation.

Create output.zip

Once you have a working solution in answer/prefixtune.py create the output.zip for upload to Coursys using:

python3 zipout.py

Create source.zip

To create the source.zip file for upload to Coursys do:

python3 zipsrc.py

You must have the following files or zipsrc.py will complain about it:

• answer/prefixtune.py – this is your solution to the homework. start by copying default.py as explained below.
• answer/prefixtune.ipynb – this is the iPython notebook that will be your write-up for the homework.

Each group member should write about what they did for this homework in the Python notebook.

Upload to Coursys

Go to Homework 2 on Coursys and do a group submission:

• Upload output.zip and source.zip
• Make sure you have documented your approach in answer/prefixtune.ipynb.
• Make sure each member of your group has documented their contribution to this homework in the Python notebook.

Grading

The grading is split up into the following components:

• dev scores (see Table below)
• test scores (see Table below)
• iPython notebook write-up
  • Make sure that you are not using any external data sources in your solution.
  • Make sure you have implemented the fine-tuning model improvements yourself without using external libraries.
• Check if each group member has written about what they did in the Python notebook.

Your BLEU score should be equal to or greater than the score listed for the corresponding marks.

The score will be normalized to the marks on Coursys for the dev and test scores.