General Preference Representation Model (GPM)
Authors (* indicates equal contribution)
Yifan Zhang*, Ge Zhang*, Yue Wu*, Kangping Xu, Quanquan Gu
As Huggingface Daily Papers: https://huggingface.co/papers/2410.02197
Code Repository: General-Preference-Model (https://github.com/general-preference/general-preference-model)
Base Model: google/gemma-2b-it
Overview
The General Preference Representation Model (GPM) improves preference-based reward modeling by embedding responses into a latent space to efficiently capture complex, intransitive human preferences. GPM achieves linear query complexity, allowing for expressive preference representation, and outperforms traditional Bradley-Terry (BT) reward models, particularly in handling cyclic preferences.
Key Features
- Preference Representation Learning: Embeds responses in a multi-dimensional latent space to model intricate human preferences, including cyclic and intransitive structures.
- Efficient Querying: Reduces computational complexity to O(K), compared to O(K²) for traditional methods, making GPM scalable for large response sets.
- General Preference Optimization (GPO): Introduces a preference score that integrates with reinforcement learning methods to optimize policy alignment with human preferences.
Evaluation
The GPM is evaluated using the RewardBench leaderboard, showing significant improvements over the BT model, with a performance margin of up to 9.11%. GPM also excels in modeling cyclic preferences, achieving 100% accuracy on cyclic datasets.
Usage
To use this model, please refer to the General Preference Model Code Repository. The repository includes detailed instructions for finetuning, evaluation, and integration of the GPM with downstream tasks. Below is an example code snippet:
from typing import Optional, List, Dict
import torch
import torch.nn as nn
from transformers import AutoConfig, AutoModel, AutoModelForCausalLM
import torch.nn.functional as F
from transformers import AutoTokenizer
import os
from safetensors.torch import load_file
from huggingface_hub import snapshot_download
def get_tokenizer(pretrain, model, padding_side="left", use_fast=True):
tokenizer = AutoTokenizer.from_pretrained(pretrain, trust_remote_code=True, use_fast=use_fast)
tokenizer.padding_side = padding_side
if tokenizer.pad_token is None:
tokenizer.pad_token = tokenizer.eos_token
tokenizer.pad_token_id = tokenizer.eos_token_id
model.config.pad_token_id = tokenizer.pad_token_id
return tokenizer
def get_reward_model(base_causal_model, base_llm_model, value_head_dim: int, add_prompt_head: bool, is_general_preference: bool=False):
class CustomRewardModel(base_causal_model):
def __init__(self, config: AutoConfig):
super().__init__(config)
setattr(self, self.base_model_prefix, base_llm_model(config))
self.is_general_preference = is_general_preference
self.value_head = nn.Linear(config.hidden_size, value_head_dim, bias=False)
if add_prompt_head:
self.prompt_head = nn.Linear(config.hidden_size, value_head_dim // 2, bias=False)
def custom_forward(
self,
input_ids: torch.LongTensor = None,
attention_mask: Optional[torch.Tensor] = None,
return_output=False,
) -> torch.Tensor:
position_ids = attention_mask.long().cumsum(-1) - 1
position_ids.masked_fill_(attention_mask == 0, 1)
outputs = getattr(self, self.base_model_prefix)(
input_ids, attention_mask=attention_mask, position_ids=position_ids
)
last_hidden_states = outputs["last_hidden_state"]
if not self.is_general_preference:
values = self.value_head(last_hidden_states).squeeze(-1)
# left padding in training mode
if self.training:
reward = values[:, -1]
else:
eos_indices = attention_mask.size(1) - 1 - attention_mask.long().fliplr().argmax(dim=1, keepdim=True)
reward = values.gather(dim=1, index=eos_indices).squeeze(1)
if return_output:
return reward, outputs
else:
return reward, None
else:
values = self.value_head(last_hidden_states)
# left padding in training mode
if self.training:
reward = values[:, -1, :]
reward = F.normalize(reward, p=2, dim=-1) # Shape will be [batch_size, value_head_dim]
else:
eos_indices = attention_mask.size(1) - 1 - attention_mask.long().fliplr().argmax(dim=1)
eos_indices = eos_indices.unsqueeze(1) # Change shape to [batch_size, 1]
reward_list = []
for dim in range(self.value_head.out_features):
reward_list.append(values[:,:,dim].gather(dim=1, index=eos_indices))
reward = torch.cat(reward_list, dim=1)
reward = F.normalize(reward, p=2, dim=-1) # Shape will be [batch_size, value_head_dim]
if return_output:
return reward, outputs
else:
return reward, None
def create_skew_symmetric_block_matrix(self, dim, device, dtype, prompt_hidden_states):
"""
Create a batch of skew-symmetric block matrices where each matrix is data-dependent on
the corresponding prompt_hidden_states. Only the relevant block diagonal parts are generated.
Args:
- dim: Dimension of the square matrix (must be even).
- prompt_hidden_states: Tensor of shape [batch_size, hidden_dim].
Returns:
- batch_R_matrices: Tensor of shape [batch_size, dim, dim], with skew-symmetric block entries.
"""
if hasattr(self, 'prompt_head'):
batch_size = prompt_hidden_states.shape[0]
# Ensure that dim is even, as we're creating blocks of size 2x2
assert dim % 2 == 0, "dim must be even for skew-symmetric block generation"
# Pass through the linear layer to get the block diagonal entries (half of the matrix's off-diagonal blocks)
block_values = self.prompt_head(prompt_hidden_states).view(batch_size, dim // 2)
block_values = torch.softmax(block_values, dim=-1)
# Create a batch of zero matrices [batch_size, dim, dim]
batch_R_matrices = torch.zeros((batch_size, dim, dim), device=device, dtype=dtype)
# Fill only the block diagonal entries with the learned values
for i in range(0, dim, 2):
batch_R_matrices[:, i, i + 1] = -block_values[:, i // 2]
batch_R_matrices[:, i + 1, i] = block_values[:, i // 2] # Skew-symmetric condition
else:
raise AttributeError("prompt_head is not defined. Ensure 'add_prompt_head' is set to True during initialization.")
return batch_R_matrices
return CustomRewardModel
def generate_high_dim_result_with_prompt(model, value_head_dim, chosen_reward, rejected_reward, prompt_hidden_states):
R_matrix = model.create_skew_symmetric_block_matrix(value_head_dim, chosen_reward.device, chosen_reward.dtype, prompt_hidden_states)
if chosen_reward.device == rejected_reward.device == R_matrix.device:
transformed_chosen = torch.bmm(chosen_reward.view(chosen_reward.shape[0], 1, value_head_dim), R_matrix.transpose(1, 2))
result = torch.bmm(transformed_chosen, rejected_reward.view(rejected_reward.shape[0], value_head_dim, 1))
result = result.view(chosen_reward.shape[0])
return result
class GPMPipeline:
def __init__(self, model_name_or_path, device=torch.device("cuda:0"), is_general_preference: bool=True, bf16: bool=True, truncation: bool=True, max_length: int=4096, padding: bool=True, tau: float=0.1):
self.device = device
self.is_general_preference = is_general_preference
self.truncation = truncation
self.max_length = max_length
self.padding = padding
self.tau = tau
config = AutoConfig.from_pretrained(model_name_or_path, trust_remote_code=True)
config._attn_implementation = "flash_attention_2"
base_class = AutoModel._model_mapping[type(config)]
base_causal_class = AutoModelForCausalLM._model_mapping.get(type(config), None)
try:
dir_path = snapshot_download(repo_id=model_name_or_path)
except Exception as e:
dir_path = model_name_or_path
combined_weights = {}
for filename in os.listdir(dir_path):
if filename.endswith(".safetensors"):
file_path = os.path.join(dir_path, filename)
weights = load_file(file_path)
combined_weights.update(weights)
if "value_head.weight" in combined_weights:
self.value_head_dim = combined_weights["value_head.weight"].shape[0]
self.add_prompt_head = True if "prompt_head.weight" in combined_weights else False
cls_class = get_reward_model(base_causal_class, base_class, add_prompt_head=self.add_prompt_head, value_head_dim=self.value_head_dim, is_general_preference=is_general_preference)
# configure model
self.model = cls_class.from_pretrained(
model_name_or_path,
config=config,
trust_remote_code=True,
torch_dtype=torch.bfloat16 if bf16 else "auto",
)
# configure tokenizer
self.tokenizer = get_tokenizer(model_name_or_path, self.model, "left", use_fast=True)
self.tokenizer.truncation_side = "right"
# prepare model
self.model.to(device)
self.model.eval()
def __call__(self, samples: List[List[Dict[str, str]]], return_prompt=False):
input_texts = [self.tokenizer.apply_chat_template(sample, tokenize=False) for sample in samples]
inputs = self.tokenizer(
input_texts,
truncation=True,
max_length=self.max_length,
padding=True,
return_tensors="pt",
).to(self.device)
inputs["input_ids"][:, -1] = self.tokenizer.eos_token_id
inputs["attention_mask"][:, -1] = 1
with torch.no_grad():
rewards, outputs = self.model.custom_forward(**inputs, return_output=return_prompt)
chosen_response_len_list = []
if return_prompt:
prompt_texts = [self.tokenizer.apply_chat_template([sample[0]], tokenize=False) for sample in samples]
for i in range(len(input_texts)):
prompt_token = self.tokenizer(
prompt_texts[i],
max_length=self.max_length,
padding=False,
truncation=True,
return_tensors="pt",
)
chosen_token = self.tokenizer(
input_texts[i],
max_length=self.max_length,
padding=False,
truncation=True,
return_tensors="pt",
)
chosen_response_len = chosen_token["attention_mask"].sum() - prompt_token["attention_mask"].sum()
chosen_response_len_list.append(chosen_response_len)
chosen_response_len = torch.tensor(chosen_response_len_list).view(-1, 1).to(self.device)
if return_prompt:
chosen_last_hidden_states = outputs["last_hidden_state"]
prompt_end_index = chosen_last_hidden_states.size(1) - chosen_response_len - 1
prompt_end_index_expanded = prompt_end_index.unsqueeze(-1).expand(-1, -1, chosen_last_hidden_states.size(-1))
prompt_hidden_state = torch.gather(chosen_last_hidden_states, dim=1, index=prompt_end_index_expanded).squeeze(1)
return rewards, prompt_hidden_state
else:
return rewards
prompt_text = "Describe the importance of reading books in today's digital age."
response1 = "Books remain crucial in the digital era, offering in-depth knowledge and fostering critical thinking. They provide a unique, immersive experience that digital media can't replicate, contributing significantly to personal and intellectual growth."
response2 = "Books are still useful for learning new things. They help you relax and can be a good break from screens."
context1 = [
{"role": "user", "content": prompt_text},
{"role": "assistant", "content": response1}
]
context2 = [
{"role": "user", "content": prompt_text},
{"role": "assistant", "content": response2}
]
rm = GPMPipeline("general-preference/GPM-Gemma-2B")
reward1, prompt_hidden_state = rm([context1], return_prompt=True)
reward2 = rm([context2])
result = generate_high_dim_result_with_prompt(rm.model, rm.value_head_dim, reward1, reward2, prompt_hidden_state)
# score = result / rm.tau
result_batch = result.float().cpu().detach().numpy().tolist()
results = []
[
results.append(1) if result > 0 else results.append(0)
for result in result_batch
]
print(result_batch)
Citation
If you find this work useful for your research, please consider citing:
@article{zhang2024general,
title={General Preference Modeling with Preference Representations for Aligning Language Models},
author={Zhang, Yifan and Zhang, Ge and Wu, Yue and Xu, Kangping and Gu, Quanquan},
journal={arXiv preprint arXiv:2410.02197},
year={2024}
}
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