DistilBERT.py

from torch import Tensor
from torch import nn
from transformers import DistilBertModel, DistilBertTokenizer
import json
from typing import Union, Tuple, List, Dict
import os
import numpy as np
import logging

class DistilBERT(nn.Module):
    """DistilBERT model to generate token embeddings.

    Each token is mapped to an output vector from DistilBERT.
    """
    def __init__(self, model_name_or_path: str, max_seq_length: int = 128, do_lower_case: bool = True):
        super(DistilBERT, self).__init__()
        self.config_keys = ['max_seq_length', 'do_lower_case']
        self.do_lower_case = do_lower_case

        if max_seq_length > 510:
            logging.warning("BERT only allows a max_seq_length of 510 (512 with special tokens). Value will be set to 510")
            max_seq_length = 510
        self.max_seq_length = max_seq_length



        self.bert = DistilBertModel.from_pretrained(model_name_or_path)
        self.tokenizer = DistilBertTokenizer.from_pretrained(model_name_or_path, do_lower_case=do_lower_case)
        self.cls_token_id = self.tokenizer.convert_tokens_to_ids([self.tokenizer.cls_token])[0]
        self.sep_token_id = self.tokenizer.convert_tokens_to_ids([self.tokenizer.sep_token])[0]

    def forward(self, features):
        """Returns token_embeddings, cls_token"""
        # DistilBERT does not use token_type_ids
        output_tokens = self.bert(input_ids=features['input_ids'], attention_mask=features['input_mask'])[0]
        cls_tokens = output_tokens[:, 0, :]  # CLS token is first token
        features.update({'token_embeddings': output_tokens, 'cls_token_embeddings': cls_tokens, 'input_mask': features['input_mask']})
        return features

    def get_word_embedding_dimension(self) -> int:
        return self.bert.config.hidden_size

    def tokenize(self, text: str) -> List[int]:
        """
        Tokenizes a text and maps tokens to token-ids
        """
        return self.tokenizer.convert_tokens_to_ids(self.tokenizer.tokenize(text))

    def get_sentence_features(self, tokens: List[int], pad_seq_length: int):
        """
        Convert tokenized sentence in its embedding ids, segment ids and mask

        :param tokens:
            a tokenized sentence
        :param pad_seq_length:
            the maximal length of the sequence. Cannot be greater than self.sentence_transformer_config.max_seq_length
        :return: embedding ids, segment ids and mask for the sentence
        """
        pad_seq_length = min(pad_seq_length, self.max_seq_length)

        tokens = tokens[:pad_seq_length]
        input_ids = [self.cls_token_id] + tokens + [self.sep_token_id]
        sentence_length = len(input_ids)

        pad_seq_length += 2  ##Add Space for CLS + SEP token


        input_mask = [1] * len(input_ids)

        # Zero-pad up to the sequence length. BERT: Pad to the right
        padding = [0] * (pad_seq_length - len(input_ids))
        input_ids += padding

        input_mask += padding

        assert len(input_ids) == pad_seq_length
        assert len(input_mask) == pad_seq_length


        return {'input_ids': np.asarray(input_ids, dtype=np.int64), 'input_mask': np.asarray(input_mask, dtype=np.int64), 'sentence_lengths': np.asarray(sentence_length, dtype=np.int64)}

    def get_config_dict(self):
        return {key: self.__dict__[key] for key in self.config_keys}

    def save(self, output_path: str):
        self.bert.save_pretrained(output_path)
        self.tokenizer.save_pretrained(output_path)

        with open(os.path.join(output_path, 'sentence_distilbert_config.json'), 'w') as fOut:
            json.dump(self.get_config_dict(), fOut, indent=2)

    @staticmethod
    def load(input_path: str):
        with open(os.path.join(input_path, 'sentence_distilbert_config.json')) as fIn:
            config = json.load(fIn)
        return DistilBERT(model_name_or_path=input_path, **config)