model.py

import json
import logging
import math
import os
import sys
from io import open

import torch
from torch import nn
from torch.nn import CrossEntropyLoss, MSELoss
import torch.nn.functional as F

from transformers.modeling_bert import BertModel, BertPreTrainedModel, BERT_START_DOCSTRING, BERT_INPUTS_DOCSTRING
from transformers.modeling_utils import PreTrainedModel, prune_linear_layer
from transformers.configuration_bert import BertConfig
from transformers.file_utils import add_start_docstrings

from torch.autograd import Variable


@add_start_docstrings("""Bert Model with a span classification head on top for extractive question-answering tasks like SQuAD (a linear layers on top of
    the hidden-states output to compute `span start logits` and `span end logits`). """,
    BERT_START_DOCSTRING, BERT_INPUTS_DOCSTRING)
class BertForQuestionAnswering(BertPreTrainedModel):
    r"""
        **start_positions**: (`optional`) ``torch.LongTensor`` of shape ``(batch_size,)``:
            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**: (`optional`) ``torch.LongTensor`` of shape ``(batch_size,)``:
            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.

    Outputs: `Tuple` comprising various elements depending on the configuration (config) and inputs:
        **loss**: (`optional`, returned when ``labels`` is provided) ``torch.FloatTensor`` of shape ``(1,)``:
            Total span extraction loss is the sum of a Cross-Entropy for the start and end positions.
        **start_scores**: ``torch.FloatTensor`` of shape ``(batch_size, sequence_length,)``
            Span-start scores (before SoftMax).
        **end_scores**: ``torch.FloatTensor`` of shape ``(batch_size, sequence_length,)``
            Span-end scores (before SoftMax).
        **hidden_states**: (`optional`, returned when ``config.output_hidden_states=True``)
            list of ``torch.FloatTensor`` (one for the output of each layer + the output of the embeddings)
            of shape ``(batch_size, sequence_length, hidden_size)``:
            Hidden-states of the model at the output of each layer plus the initial embedding outputs.
        **attentions**: (`optional`, returned when ``config.output_attentions=True``)
            list of ``torch.FloatTensor`` (one for each layer) of shape ``(batch_size, num_heads, sequence_length, sequence_length)``:
            Attentions weights after the attention softmax, used to compute the weighted average in the self-attention heads.

    Examples::

        tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
        model = BertForQuestionAnswering.from_pretrained('bert-large-uncased-whole-word-masking-finetuned-squad')
        question, text = "Who was Jim Henson?", "Jim Henson was a nice puppet"
        input_text = "[CLS] " + question + " [SEP] " + text + " [SEP]"
        input_ids = tokenizer.encode(input_text)
        token_type_ids = [0 if i <= input_ids.index(102) else 1 for i in range(len(input_ids))] 
        start_scores, end_scores = model(torch.tensor([input_ids]), token_type_ids=torch.tensor([token_type_ids]))
        all_tokens = tokenizer.convert_ids_to_tokens(input_ids)  
        print(' '.join(all_tokens[torch.argmax(start_scores) : torch.argmax(end_scores)+1]))
        # a nice puppet


    """
    def __init__(self, config):
        super(BertForQuestionAnswering, self).__init__(config)
        self.num_labels = config.num_labels

        self.bert = BertModel(config)
        self.qa_outputs = nn.Linear(config.hidden_size, config.num_labels)
        self.selected_outputs = nn.Linear(config.hidden_size, 1)

        self.init_weights()

    def forward(self, input_ids, attention_mask=None, token_type_ids=None, position_ids=None, head_mask=None):

        outputs = self.bert(input_ids,
                            attention_mask=attention_mask,
                            token_type_ids=token_type_ids,
                            position_ids=position_ids, 
                            head_mask=head_mask)

        sequence_output = outputs[0]
        pooled_output = outputs[1]

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

        outputs = (start_logits, end_logits, selected_logits,) + outputs[2:]
        return outputs