modeling.py

"""
BertRegressor — truncated bert-base-uncased + single-Linear regression head.

Architecture used in the Ace-CEFR baseline reproduction
(https://arxiv.org/abs/2506.14046, §4.5.1).

The model loads the first `num_hidden_layers` transformer blocks of
`bert-base-uncased`, plus its embeddings and pooler, and predicts a CEFR
difficulty score as a float in [1.0, 6.0] (A1 = 1, A2 = 2, B1 = 3, B2 = 4,
C1 = 5, C2 = 6).

Example:
    >>> import torch
    >>> from transformers import BertTokenizerFast
    >>> from modeling import BertRegressor
    >>> model = BertRegressor("bert-base-uncased", num_layers=3)
    >>> sd = torch.load("pytorch_model.bin", map_location="cpu")
    >>> model.load_state_dict(sd)
    >>> model.eval()
    >>> tok = BertTokenizerFast.from_pretrained("bert-base-uncased")
    >>> enc = tok(["Hello, how are you?"], return_tensors="pt",
    ...           padding="max_length", truncation=True, max_length=128)
    >>> with torch.no_grad():
    ...     score = model(enc["input_ids"], enc["attention_mask"],
    ...                   enc["token_type_ids"]).clamp(1.0, 6.0).item()
    >>> print(score)  # e.g. 1.4
"""

import torch
import torch.nn as nn
from transformers import BertConfig, BertModel


class BertRegressor(nn.Module):
    def __init__(self, model_name: str = "bert-base-uncased", num_layers: int = 3):
        super().__init__()
        cfg = BertConfig.from_pretrained(model_name)
        cfg.num_hidden_layers = num_layers
        self.bert = BertModel(cfg)

        pretrained = BertModel.from_pretrained(model_name)
        self.bert.embeddings.load_state_dict(pretrained.embeddings.state_dict())
        for i in range(num_layers):
            self.bert.encoder.layer[i].load_state_dict(
                pretrained.encoder.layer[i].state_dict()
            )
        self.bert.pooler.load_state_dict(pretrained.pooler.state_dict())
        del pretrained

        self.regressor = nn.Linear(cfg.hidden_size, 1)

    def forward(self, input_ids, attention_mask, token_type_ids):
        out = self.bert(
            input_ids=input_ids,
            attention_mask=attention_mask,
            token_type_ids=token_type_ids,
        )
        return self.regressor(out.pooler_output).squeeze(-1)