---
license: apache-2.0
base_model:
- google/gemma-2-2b-it
---

# General Preference Representation Model (GPM)

+ **Authors** (* indicates equal contribution)

    Yifan Zhang*, Ge Zhang*, Yue Wu*, Kangping Xu, Quanquan Gu

+ **Paper**: [General Preference Modeling with Preference Representations for Aligning Language Models (https://arxiv.org/abs/2410.02197)](https://arxiv.org/abs/2410.02197)
+ **As Huggingface Daily Papers**: [https://huggingface.co/papers/2410.02197](https://huggingface.co/papers/2410.02197)
+ **Code Repository**: [General-Preference-Model (https://github.com/general-preference/general-preference-model)](https://github.com/general-preference/general-preference-model)
+ **Dataset**: [natolambert/skywork-preferences-80k-v0.1-cleaned](https://huggingface.co/datasets/natolambert/skywork-preferences-80k-v0.1-cleaned)
+ **Base Model**: [google/gemma-2-2b-it](https://huggingface.co/google/gemma-2-2b-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](https://github.com/allenai/reward-bench) leaderboard, showing significant improvements over the BT model, with a performance margin of up to 2.31%. 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](https://github.com/general-preference/general-preference-model). The repository includes detailed instructions for finetuning, evaluation, and integration of the GPM with downstream tasks. Below is an example code snippet:

```python
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-2-2B")

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}
}
```