ULTS: Uncertainty-guided Likelihood Tree Search

Accompanying implementation of the following paper [ArXiv]:

@inproceedings{grosse2024ults,
  title={Uncertainty-Guided Optimization On Large Language Model Search Trees},
  author={Grosse, Julia and Wu, Ruotian and Rashid, Ahmad and Hennig, Philipp and Poupart, Pascal and Kristiadi, Agustinus},
  booktitle={Sixth Symposium on Advances in Approximate Bayesian Inference --- Non Archival Track},
  year={2024}
}

Setup

Requires Python >= 3.9.

1. Install PyTorch with CUDA, version >= 2.0
2. Install this package: pip install ults

Usage

See full example here: examples/generate.py.

Quickstart with the Dirichlet prior

Important

ULTS will first check prior_dir directory (default ./ults_priors) for a precomputed prior with your choices of width (vocab size), depth (max tokens to generate), and $\alpha$ (concentration strength). If not exists, then it will compute and cache the prior --- this might take a while! However, this only needs to be done once for each each of the choices above. In the subsequent generation call, the decoding will be very quick.

tokenizer = AutoTokenizer.from_pretrained("meta-llama/Llama-2-7b-hf")
model = AutoModelForCausalLM.from_pretrained(
  "meta-llama/Llama-2-7b-hf", torch_dtype=torch.bfloat16
)
model.eval()

text = "Moose is a"
model_inputs = tokenizer(text, return_tensors="pt")

-output = model.generate(
-    **model_inputs,
-    num_beams=5,
-    max_new_tokens=40,
-)
-generated_sequence = output.sequences

+import ults
+output = ults.generate(
+    model=model,
+    model_inputs=model_inputs,
+    max_tokens=40,
+)
+generated_sequence = output.sequence

generated_text = tokenizer.decode(generated_sequence[0])

Using the Empirical Prior

On top of the default Dirichlet priors (agnostic to the LLM), ULTS can also leverage empirical priors, specific to the LLM at hand. Example precomputed empirical priors, compatible with Llama-2-7b, Mistral-7B-v0.1, and Gemma-7b, are available in examples/ults_priors.

1. First, gather samples of the LLM's softmax outputs from different time steps. Here we will use the greedy decoding. See examples/sample_llm_outputs.py for a complete example

RESULT_DIR = f"./ults_priors/llm_output_samples/{DATASET_NAME}_{LLM_NAME}"

# Samples of contexts from your dataset
contexts: List[str]

for idx, context in enumerate(contexts):
    input_ids = tokenizer(sentence[sent_key], return_tensors="pt")["input_ids"]

    # `n_tokens` is the max. depth of the tree that you want to optimize on
    # i.e., the max number of tokens you want to generate with ULTS
    for d in range(n_tokens):
        with torch.no_grad():
            outputs = torch.softmax(model(input_ids).logits, dim=-1)

        # Save the last softmax output (this is our empirical sample for depth `d`)
        outputs = outputs[0, -1, :]
        torch.save(outputs, f"{RESULT_DIR}/sample_index{idx}_depth{d}.pt")

        # Continue greedy generation
        index = torch.argmax(qualities)
        model_input = torch.cat([model_input, index.expand(1, 1)], dim=1)

# Stack them together into a (n_samples*n_tokens, vocab_size) tensor
import glob, random
sample_files = glob.glob(f"{RESULT_DIR}/sample_*.pt")
samples = [torch.load(sample) for sample in sample_files]
torch.save(torch.vstack(samples), f'{RESULT_DIR}/all_samples.pt')

2. Then, when specify the prior when calling ULTS. Everything else stays the same as in examples/generate.py.

output = ults.generate(
    ...
+   prior_kind="empirical",
+   prior_empirical_llm_samples=torch.load(f'{RESULT_DIR}/all_samples.pt')
    ...
)

Caveats

1. Currently doesn't support batch generation.
2. Huggingface optimizes the average log-likelihood. It is effectively penalizes shorter sequences. Meanwhile, ULTS optimizes the total log-likelihood, so the behavior differs from Huggingface's. There is a plan to support this in ULTS, see #36.

Development

This repo uses pdm as the dependency manager and the build system.

1. Install pdm, see: https://pdm-project.org/en/latest/
2. Run pdm install

All dependencies will then be installed by pdm. Moreover the current repo will be installed in an editable mode.

Important

Before pushing your code, ensure that all tests pass and all linting and formatting issues are resolved.

1. Run pytest and make sure all tests pass.
2. Run make ruff and ensure:
   1. All codes are formatted correctly.
   2. There is no linting issue.