reward_fn.py

import torch
from transformers import AutoTokenizer, AutoModelForSequenceClassification

# Load reward model once
rm_name = "OpenAssistant/reward-model-deberta-v3-large-v2"

rm_tokenizer = AutoTokenizer.from_pretrained(rm_name)
rm_model = AutoModelForSequenceClassification.from_pretrained(
    rm_name,
    torch_dtype=torch.float32
).eval()

# For Spaces: keep reward model on CPU to save VRAM
device = torch.device("cpu")
rm_model.to(device)


def reward_fn(text: str) -> float:
    """
    Returns a scalar helpfulness reward from a trained reward model.
    Higher = more helpful, clearer, more aligned response.
    """
    inputs = rm_tokenizer(
        text,
        return_tensors="pt",
        truncation=True,
        max_length=512
    ).to(device)

    with torch.no_grad():
        scores = rm_model(**inputs).logits

    # Reward = model's score for "helpful" class
    reward = float(scores[0].item())

    return reward