modelling.py

from typing import Optional, Tuple, Union

import torch
from torch import nn
from torch.nn import CrossEntropyLoss
from transformers import ModernBertModel, ModernBertPreTrainedModel, ModernBertConfig
from transformers.modeling_outputs import QuestionAnsweringModelOutput
from transformers.models.modernbert.modeling_modernbert import ModernBertPredictionHead


class ModernBertForQuestionAnswering(ModernBertPreTrainedModel):
    def __init__(self, config: ModernBertConfig):
        super().__init__(config)
        self.num_labels = config.num_labels
        self.config = config

        self.model = ModernBertModel(config)
        self.head = ModernBertPredictionHead(config)
        self.qa_outputs = nn.Linear(config.hidden_size, 2)  # 2 for start/end position logits
        self.qa_outputs.weight.data.normal_(mean=0.0, std=0.02)
        self.qa_outputs.bias.data.zero_()

        self.drop = torch.nn.Dropout(config.classifier_dropout)

        # Initialize weights and apply final processing
        self.post_init()

    def forward(
            self,
            input_ids: Optional[torch.Tensor],
            attention_mask: Optional[torch.Tensor] = None,
            sliding_window_mask: Optional[torch.Tensor] = None,
            position_ids: Optional[torch.Tensor] = None,
            start_positions: Optional[torch.Tensor] = None,
            end_positions: Optional[torch.Tensor] = None,
            indices: Optional[torch.Tensor] = None,
            cu_seqlens: Optional[torch.Tensor] = None,
            max_seqlen: Optional[int] = None,
            batch_size: Optional[int] = None,
            seq_len: Optional[int] = None,
            output_attentions: Optional[bool] = None,
            output_hidden_states: Optional[bool] = None,
            return_dict: Optional[bool] = None,
            **kwargs,
    ) -> Union[Tuple[torch.Tensor], QuestionAnsweringModelOutput]:
        r"""
        start_positions (`torch.LongTensor` of shape `(batch_size,)`, *optional*):
            Labels for position (index) of the start of the labelled span for computing the token classification loss.
            Positions are clamped to the length of the sequence (`sequence_length`). Position outside of the sequence
            are not taken into account for computing the loss.
        end_positions (`torch.LongTensor` of shape `(batch_size,)`, *optional*):
            Labels for position (index) of the end of the labelled span for computing the token classification loss.
            Positions are clamped to the length of the sequence (`sequence_length`). Position outside of the sequence
            are not taken into account for computing the loss.
        """
        return_dict = return_dict if return_dict is not None else self.config.use_return_dict
        self._maybe_set_compile()

        outputs = self.model(
            input_ids,
            attention_mask=attention_mask,
            sliding_window_mask=sliding_window_mask,
            position_ids=position_ids,
            indices=indices,
            cu_seqlens=cu_seqlens,
            max_seqlen=max_seqlen,
            batch_size=batch_size,
            seq_len=seq_len,
            output_attentions=output_attentions,
            output_hidden_states=output_hidden_states,
            return_dict=True,
        )

        sequence_output = outputs[0]
        sequence_output = self.drop(self.head(sequence_output))

        logits = self.qa_outputs(sequence_output)
        start_logits, end_logits = logits.split(1, dim=-1)
        start_logits = start_logits.squeeze(-1).contiguous()
        end_logits = end_logits.squeeze(-1).contiguous()

        total_loss = None
        if start_positions is not None and end_positions is not None:
            # If we are on multi-GPU, split add a dimension
            if len(start_positions.size()) > 1:
                start_positions = start_positions.squeeze(-1)
            if len(end_positions.size()) > 1:
                end_positions = end_positions.squeeze(-1)
            # sometimes the start/end positions are outside our model inputs, we ignore these terms
            ignored_index = start_logits.size(1)
            start_positions = start_positions.clamp(0, ignored_index)
            end_positions = end_positions.clamp(0, ignored_index)

            loss_fct = CrossEntropyLoss(ignore_index=ignored_index)
            start_loss = loss_fct(start_logits, start_positions)
            end_loss = loss_fct(end_logits, end_positions)
            total_loss = (start_loss + end_loss) / 2

        if not return_dict:
            output = (start_logits, end_logits) + outputs[2:]
            return ((total_loss,) + output) if total_loss is not None else output

        return QuestionAnsweringModelOutput(
            loss=total_loss,
            start_logits=start_logits,
            end_logits=end_logits,
            hidden_states=outputs.hidden_states,
            attentions=outputs.attentions,
        )