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🕺🏽 disco: A Toolkit for Distributional Control of Generative Models

The 🕺🏽 disco toolkit allows to control language models and other generative systems to match human preferences while avoiding catastrophic forgetting.

To achieve this, disco decouples the problem of expressing what properties the model should have from how to actually get the desired model as separate steps.

Step 1: ⚓ We express how the target distribution should be

First, we define some feature over the generated samples that matters to us. It can be anything we can compute. For example, on a language model it can be as simple as whether the generated text contains a certain word or as complex as the compilability of some generated piece of code. Importantly, there is no need for the feature to be differentiable.

Then, we can express our preferences on the target distribution by deciding how prevalent the feature should be. For example, we might want to ask that a certain word appears 50% of the time when sampling from the model; or that 100% of the generated code is compilable. The resulting target distribution is expressed as an energy-based model or EBM, which is an unnormalized probability distribution that respects the desired moments while avoiding catastrophic forgetting, as a result of having minimal KL divergence to the original model.

The resulting representation of the target distribution can score samples, but cannot directly be used to generate them.

Step 2: 🎯 Approximate the target distribution

To generate samples from the target distribution we can tune a model to approximate it. We do this by minimizing the divergence to the target distribution. While techniques such as reinforcement learning from human feedback (RLHF) are restricted to using one kind of divergence only (specifically, reverse KL divergence), disco is more general, allowing the use of the full class of f-divergences, including both forward and reverse KL divergence, Jensen-Shannon, and total variation distance.

Step 3: 💬 Generate content that matches the preferences

The resulting model can generate samples directly from a close approximation of the target distribution. Furthermore, it can be used jointly with Quasi-Rejection Sampling (QRS), a Monte Carlo sampling technique that allows the generation of samples that are even more representative of the target distribution. Alternatively, it is then possible to use decoding methods such as nucleus sampling, top-k sampling, or beam search, which would return samples from a further updated target distribution.

See the references below for more theoretical and technical details.

Installation

Standard installation

The easiest way to install disco is to rely on pip, asking for the disco-generation package:

pip install disco-generation

Note that the toolkit:

  • depends on PyTorch,
  • uses HuggingFace's Transformers library for the generic handling of language models, as well as the generation of samples from them.

Toolkit Developers

If we plan to extend the toolkit we will need to clone and to install it as a local package. From the toolkit top folder, once we've git-cloned the repository and activated our development environment, we simply do:

pip install -e .

Quick introduction

Distributions

The generative model that we want to tune must be wrapped by a Distribution object. For example, for a (causal or seq2seq) language model compatible with the 🤗 Hugging Face interface use an LMDistribution.

A valid Distribution must have the following two methods:

  • .sample(context) that given an optional context on which the distribution can be conditioned, returns a list of samples from the underlying distribution and a tensor with their corresponding log-probabilities;
  • .log_score(samples, context) that given a list of samples and the context on which to condition the distribution, returns their corresponding log-probabilities.
from disco.distributions import LMDistribution
distribution = LMDistribution()

incipit = "It was a cold and stormy night"
samples, log_scores = distribution.sample(context=incipit)

distribution.log_score(samples, context=incipit)

LMDistribution generate samples, with the TextSample type, which are named tuples with both a text and token_ids fields.

From now on, after this initial example, imports will be skipped for clarity.

Features

Features are represented by an object with the method

  • .score(samples, context) which given a list of samples and an eventual context returns a tensor of real-valued scores.

A convenient way to define one is using the Scorer class, which accepts a function or a lambda abstraction that takes sample and a context, and vectorizes it. For example, we can compute the effective length of a GPT-2 text sample by finding the eos token:

sequence_length = Scorer(lambda s, c: s.text.index("<|endoftext|>"))

Where s is the sample (assumed to be a TextSample) and c is an eventual context.

Boolean Features

An important class of features are boolean features. While general features can only be used to define distributional constraints, boolean features can also be used to define pointwise constraints, see below. To define one, we can use the BooleanScorer helper class, which takes a function as an argument. For example, we can score the presence of the string "amazing", as follows:

amazing = BooleanScorer(lambda s, c: "amazing" in s.text)

The False/True results from the lambda are casted to 0.0/1.0 float values so that they can be used in the EBM definition.

BooleanScorer belongs to the more general PositiveScorer class, which can be used to construct EBMs. The main properties of a PostiveScorer are that first, it returns positive scorers, and second that it provides the method:

  • .log_score(samples, context) that given a list of samples and the context on which to condition the distribution, returns their corresponding log-probabilities.

As a consequence, we can see that a Distribution is also a PositiveScorer that is able to sample as well.

Controlling Generation

Expressing preferences through an EBM

We express preferences over the distribution by defining target moments for specific features. This results in a target distribution that matches the desired moments while minimizing the KL divergence to the original distribution. In other words, it incorporates the preferences while avoiding catastrophic forgetting. This distribution is represented as an EBM, which can be used to score samples, in other words it is a PositiveScorer, but cannot be used to sample, we'll see how to sample below.

We can express either pointwise or distributional constraints on a distribution and compose them at will. The former expresses a (boolean) property that must apply to all sequences, whereas the latter represents properties at the distributional level.

To obtain the target distribution that incorporates our constraints, we use the constraint method of the corresponding Distribution. This method takes a list of features and their corresponding target moments.

For example, we can define an EBM with a pointwise constraint requiring that all our samples must include "amazing" by setting the target moment to 1 on a BooleanFeature:

target_ebm = base.constrain([amazing], [1])

Or we can ask for a distributional constraint requiring that half of the samples include "amazing":

target_ebm = base.constrain([amazing], [1/2])

Approximating the target EBM

Given an EBM target distribution, we now want to train a model to approximate it so that we can use it to generate samples. In the unconditional case, namely when there is a single fixed context used in generation, then we can use a Tuner, more specifically a DPGTuner, as follows.

target_ebm = base.constrain([amazing], [1])

model = LMDistribution(freeze=False)
incipit = "It was a cold and stormy night"

tuner = DPGTuner(model, target_ebm, context=incipit)
tuner.tune()

And we can sample amazing sequences from the tuned model.

samples, log_scores = model.sample(context=incipit)
for s in samples:
  print(incipit + s.text)
Tuning parameters

Important parameters of the Tuner include:

  • n_gradient_steps: number of total gradient steps in the full tuning process;
  • n_samples_per_step: total number of samples used in performing a gradient step (aka batch size);
  • scoring_size: number of samples sent in a batch to the .score function. This parameter affects training speed or helps solve GPU memory errors, but does not affect final results;
  • sampling_size: number of samples obtained from a single call to the .sample function. This parameter affects training speed or helps solve GPU memory errors, but does not affect final results;
  • features: list of pairs (name, feature) so that the feature moments will be computed by importance sampling (and reported using the key given by name);
  • track_divergence_from_base: set to True to track the reverse KL divergence from the original model —this requires an additional round of samples' scoring).

Logging

The Tuner reports a number of metrics that are useful to monitor the training progress. A number of Logger classes are provided to keep track of these metrics. Basic logging is provided though the console, as follows:

console_logger = ConsoleLogger(tuner)

However, more detailed statistics can be kept trhough a JSON/WandB/Neptune loggers:

project = "example_project"
name = "run_01"
json_logger = JSONLogger(tuner, project, name)
neptune_logger = NeptuneLogger(tuner, project, name)
wandb_logger = WandBLogger(tuner, project, name)

where project and name refer to the project and run name, respectively.

Logged Metrics

Loggers store a number of metrics about the training process. Here we list a few of the most relevant ones:

  • kl_target_model and kl_target_proposal: estimates of the forward KL divergence to the target EBM from the tuned model and the proposal distribution, respectively. In the case of using online training, the two are equivalent with the only caveat that kl_target_model is computed —this is the metric being optimized, and not the value reported as loss;
  • kl_model_base and kl_proposal_base: estimates of the reverse KL divergence to the original model of the tuned model and the proposal distribution, respectively —only reported if track_divergence_from_base is set to True;
  • Feature moments: Estimate of the features' moments for those features specified with the features parameter at the Tuner's construction time.

Controlled Conditional Generation

The conditional case is superficially very similar, with an extra step needed to instantiate a ContextDistribution, which allows to sample contexts that can then be used to condition the model. Furthermore, we use the more general CDPGTuner class.

Assuming we have a file of incipits, one per line, in a data/incipits.txt file, we could do:

target_ebm = base.constrain([amazing], [1])

model = LMDistribution(freeze=False)

tuner = CDPGTuner(model, target_ebm,
  context_distribution=ContextDistribution("data/incipits.txt"),
  context_sampling_size=2**3)
tuner.tune()

Note that while we have used a decoder-only model here for illustrative purposes, the real power of the CDPGTuner is that it allows to control seq2seq models such as those used in NMT, summarization, etc... Please refer to the dedicated tutorial notebook for an example of how to control an actual conditional model.

Improving the approximation through minimizing other f-divergences

The Tuner classes train the model by minimizing its divergence from the distribution induced by the target ebm. While in the original DPG and CDPG algorithms this divergence was always the KL divergence, recent work has generalized them to the wider class of f-divergences. To pick the loss to minimize, use the corresponding FDPGTuner or FCDPGTuner class, depending on whether you are in the unconditional or conditional case, and choose the divergence to minimize through the loss parameter. Some of the possible losses are:

  • KLLoss(): KL divergence
  • ReverseKLLoss(): KL divergence reversing the order of the arguments
  • TVLoss(): Total Variation Distance
  • JSLoss(): Jensen-Shannon divergence.

Each of these divergences strikes a different balance between level of alignment and diversity. KL exhibits lower alignment than other losses, but higher diversity. On the other hand, ReverseKL tends to produce hight alignment at the cost of lower diversity. Jensen-Shannon strikes a good balance between the two, making it a good default choice.

As an example,

target_ebm = base.constrain([amazing], [1])

model = LMDistribution(freeze=False)

tuner = FCDPGTuner(model, target_ebm, loss=JSLoss(),
  context_distribution=ContextDistribution("data/incipits.txt"),
  context_sampling_size=2**3)
tuner.tune()

Reinforcement Learning from Human Feedback (RLHF)

RLHF is a popular paradigm for aligning language models to preferences. While RLHF is commonly known in the form of a reward maximization algorithm, recent work has shown that it is equivalent to a distribution approximation problem which can be easily handled by disco. Specifically, given a reward function r(x) and the regularization parameter beta, the following code optimizes the same objective as RLHF:

target_ebm = base * ExponentialScorer([r], [1./beta])

model = base.clone()


tuner = FCDPGTuner(model, target_ebm, loss=ReverseKLLoss())

In other words, RLHF optimizes the reverse KL to the above-defined target EBM. Interestingly, this opens new opportunities as the divergence to be minimized could be now chosen to be any other, as explored here.

Monte-Carlo sampling to improve the approximation

After the tuning is done, model is now a better approximation to the target EBM, but it is not guaranteed to perfectly match this distribution. While further training can improve the situation, another alternative is using quasi-rejection sampling (QRS), a Monte-Carlo sampling technique that allows to trade-off sampling efficiency for a higher fidelity to the target distribution —a higher value of beta yields a better fidelity although at a higher computational cost.

beta=0.5
sampler = QuasiRejectionSampler(target_ebm, model, beta=beta)
samples, log_scores = sampler.sample(sampling_size=2**7)

In summary

To put some of this (distributional constraint, tuning in the unconditional case and using QRS) together:

base = LMDistribution()
target_ebm = base.constrain([amazing], [1/2])

model = LMDistribution(freeze=False)

tuner = DPGTuner(model, target_ebm)
tuner.tune()

beta=0.5
sampler = QuasiRejectionSampler(target_ebm, model, beta=beta)
samples, log_scores = sampler.sample(context=incipit, sampling_size=2**7)

Going further

A few things to keep in mind while reading the following paragraphs showing the principles of disco:

  1. this is only an introduction, skipping some details and relying on toyish use cases;
  2. the notebooks in the tutorials folder go in more depth, on more use cases;
  3. the focus here and in most notebooks is on natural language, but again the toolkit can be used to control distributions over sequences such as code or chess moves, or even other data types, as long as they respect the basic assumptions of a disco Distribution object.

References

The disco toolkit implements the theoretical framework presented in the following works:

To cite disco, please use:

@inproceedings{kruszewski-etal-2023-disco,
    title = "disco: a toolkit for Distributional Control of Generative Models",
    author = "Kruszewski, Germ{\'a}n  and
      Rozen, Jos  and
      Dymetman, Marc",
    booktitle = "Proceedings of the 61st Annual Meeting of the Association for Computational Linguistics (Volume 3: System Demonstrations)",
    month = jul,
    year = "2023",
    address = "Toronto, Canada",
    publisher = "Association for Computational Linguistics",
    url = "https://aclanthology.org/2023.acl-demo.14",
    pages = "144--160",
    abstract = "Pre-trained language models and other generative models have revolutionized NLP and beyond. However, these models tend to reproduce undesirable biases present in their training data. Also, they may overlook patterns that are important but challenging to capture. To address these limitations, researchers have introduced distributional control techniques. These techniques, not limited to language, allow controlling the prevalence (i.e. expectations) of any features of interest in the model{'}s outputs. Despite their potential, the widespread adoption of these techniques has been hindered by the difficulty in adapting the complex, disconnected code. Here, we present disco, an open-source Python library that brings these techniques to the broader public",
}

License

See LICENSE file.