CharacterBERT

This is the code repository for the paper "CharacterBERT: Reconciling ELMo and BERT for Word-LevelOpen-Vocabulary Representations From Characters" that came out at COLING 2020.

2021-02-25: Code for pre-training BERT and CharacterBERT is now available here!

Table of contents

• Paper summary
  • TL;DR
  • Motivations
• How do I use CharacterBERT?
  • Installation
  • Pre-trained models
  • Using CharacterBERT in practice
• How do I pre-train CharacterBERT?
• How do I reproduce the paper's results?
• Running experiments on GPUs
• References

Paper summary

TL;DR

CharacterBERT is a variant of BERT that produces contextual representations at the word level.

This is achieved by attending to the characters of each input token and dynamically building token representations from that. In fact, contrary to standard BERT--which relies on a matrix of pre-defined wordpieces, this approach uses a CharacterCNN module, similar to ELMo, that can generate representations for arbitrary input tokens.

The figure above shows how context-independent representations are built in BERT, vs. how they are built in CharacterBERT. Here, we assume that "Apple" is an unknown token, which results in BERT splitting the token into two wordpieces "Ap" and "##ple" and embedding each unit. On the other hand, CharacterBERT processes the token "Apple" as is, then attends to its characters in order to produce a single token embedding.

Motivations

CharacterBERT has two main motivations:

• It is frequent to adapt the original BERT to new specialized domains (e.g. medical, legal, scientific domains..) by simply re-training it on a set of specialized corpora. This results in the original (general domain) wordpiece vocabulary being re-used despite the final model being actually targeted toward a different potentially highly specialized domain, which is arguably suboptimal (see Section 2 of the paper).

  A straightforward solution in this case would be to train a new BERT from scratch with a specialized wordpiece vocabulary. However, training a single BERT is already costly enough let alone needing to train one for each and every domain of interest.

• BERT uses a wordpiece system to strike a good balance between the specificity of tokens and flexibility of characters. However, working with subwords is not the most convenient in practice (should we average the representations to get the original token embedding for word similarity tasks? should we only use the first wordpiece of each token in sequence labelling tasks? ...) and most would just prefer to work with tokens.

Inspired by ELMo, we use a CharacterCNN module and manage to get a variant of BERT that produces both word-level and contextual representations which can also be re-adapted as many times as necessary, on any domain, without needing to worry about the suitability of any wordpieces. And as a cherry on top, attending to the characters of each input token also leads us to a more robust model against any typos or misspellings (see Section 5.5 of the paper).

How do I use CharacterBERT?

Installation

We recommend using a virtual environment that is specific to using CharacterBERT.

If you do not already have conda installed, you can install Miniconda from this link. Then, check that your conda is up to date:

conda update -n base -c defaults conda

Create a fresh conda environment:

conda create python=3.10 --name=character-bert

If not already activated, activate the new conda environment using:

conda activate character-bert

Then install the following packages:

conda install pytorch cudatoolkit=11.8 -c pytorch
pip install transformers==4.34.0 scikit-learn==1.3.1 gdown==4.7.1

Note 1: If you will not be running experiments on a GPU, install pyTorch via this command instead:
conda install pytorch cpuonly -c pytorch

Note 2: If you just want to be able to load pre-trained CharacterBERT weigths, you do not have to install scikit-learn which is only used for computing Precision, Recall, F1 metrics during evaluation.

Pre-trained models

You can use the download.py script to download any of the models below:

^{1, 2} We offer BERT models as well as CharacterBERT models since we have pre-trained both architectures in an effort to fairly compare these architectures. Our BERT models use the same architecture and starting wordpiece vocabulary as bert-base-uncased.

For instance, let's download the medical version of CharacterBERT:

python download.py --model='medical_character_bert'

We can download also download all models in a single command:

python download.py --model='all'

Using CharacterBERT in practice

CharacterBERT's architecture is almost identical to BERT's, so you can easilly adapt any code that from the Transformers library.

Example 1: getting word embeddings from CharacterBERT

"""Basic example: getting word embeddings from CharacterBERT"""
from transformers import BertTokenizer
from modeling.character_bert import CharacterBertModel
from utils.character_cnn import CharacterIndexer

# Example text
x = "Hello World!"

# Tokenize the text
tokenizer = BertTokenizer.from_pretrained(
    './pretrained-models/bert-base-uncased/')
x = tokenizer.basic_tokenizer.tokenize(x)

# Add [CLS] and [SEP]
x = ['[CLS]', *x, '[SEP]']

# Convert token sequence into character indices
indexer = CharacterIndexer()
batch = [x]  # This is a batch with a single token sequence x
batch_ids = indexer.as_padded_tensor(batch)

# Load some pre-trained CharacterBERT
model = CharacterBertModel.from_pretrained(
    './pretrained-models/medical_character_bert/')

# Feed batch to CharacterBERT & get the embeddings
embeddings_for_batch, _ = model(batch_ids)
embeddings_for_x = embeddings_for_batch[0]
print('These are the embeddings produces by CharacterBERT (last transformer layer)')
for token, embedding in zip(x, embeddings_for_x):
    print(token, embedding)

Example 2: using CharacterBERT for binary classification

""" Basic example: using CharacterBERT for binary classification """
from transformers import BertForSequenceClassification, BertConfig
from modeling.character_bert import CharacterBertModel

#### LOADING BERT FOR CLASSIFICATION ####

config = BertConfig.from_pretrained('bert-base-uncased', num_labels=2)  # binary classification
model = BertForSequenceClassification(config=config)

model.bert.embeddings.word_embeddings  # wordpiece embeddings
>>> Embedding(30522, 768, padding_idx=0)

#### REPLACING BERT WITH CHARACTER_BERT ####

character_bert_model = CharacterBertModel.from_pretrained(
    './pretrained-models/medical_character_bert/')
model.bert = character_bert_model

model.bert.embeddings.word_embeddings  # wordpieces are replaced with a CharacterCNN
>>> CharacterCNN(
        (char_conv_0): Conv1d(16, 32, kernel_size=(1,), stride=(1,))
        (char_conv_1): Conv1d(16, 32, kernel_size=(2,), stride=(1,))
        (char_conv_2): Conv1d(16, 64, kernel_size=(3,), stride=(1,))
        (char_conv_3): Conv1d(16, 128, kernel_size=(4,), stride=(1,))
        (char_conv_4): Conv1d(16, 256, kernel_size=(5,), stride=(1,))
        (char_conv_5): Conv1d(16, 512, kernel_size=(6,), stride=(1,))
        (char_conv_6): Conv1d(16, 1024, kernel_size=(7,), stride=(1,))
        (_highways): Highway(
        (_layers): ModuleList(
            (0): Linear(in_features=2048, out_features=4096, bias=True)
            (1): Linear(in_features=2048, out_features=4096, bias=True)
        )
        )
        (_projection): Linear(in_features=2048, out_features=768, bias=True)
    )

#### PREPARING RAW TEXT ####

from transformers import BertTokenizer
from utils.character_cnn import CharacterIndexer

text = "CharacterBERT attends to each token's characters"
bert_tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
tokenized_text = bert_tokenizer.basic_tokenizer.tokenize(text) # this is NOT wordpiece tokenization

tokenized_text
>>> ['characterbert', 'attends', 'to', 'each', 'token', "'", 's', 'characters']

indexer = CharacterIndexer()  # This converts each token into a list of character indices
input_tensor = indexer.as_padded_tensor([tokenized_text])  # we build a batch of only one sequence
input_tensor.shape
>>> torch.Size([1, 8, 50])  # (batch_size, sequence_length, character_embedding_dim)

#### USING CHARACTER_BERT FOR INFERENCE ####

output = model(input_tensor, return_dict=False)[0]
>>> tensor([[-0.3378, -0.2772]], grad_fn=<AddmmBackward>)  # class logits

For more complete (but still illustrative) examples you can refer to the run_experiments.sh script which runs a few Classification/SequenceLabelling experiments using BERT/CharacterBERT.

bash run_experiments.sh

You can adapt the run_experiments.sh script to try out any available model. You should also be able to add real classification and sequence labelling tasks by adapting the data.py script.

Running experiments on GPUs

In order to use GPUs you will need to make sure the PyTorch version that is in your conda environment matches your machine's configuration. To do that, you may want to run a few tests.

Let's assume you want to use the GPU n°0 on your machine. Then set:

export CUDA_VISIBLE_DEVICES=0

And run these commands to check whether pytorch can detect your GPU:

import torch
print(torch.cuda.is_available())  # Should return `True`

If the last command returns False, then there is probably a mismatch between the installed PyTorch version and your machine's configuration. To fix that, run nvidia-smi in your terminal and check your driver version:

Then compare this version with the numbers given in the NVIDIA CUDA Toolkit Release Notes:

In this example the shown version is 390.116 which corresponds to CUDA 9.0. This means that the appropriate command for installing PyTorch is:

conda install pytorch cudatoolkit=9.0 -c pytorch

Now, everything should work fine!

References

Please cite our paper if you use CharacterBERT in your work:

@inproceedings{el-boukkouri-etal-2020-characterbert,
    title = "{C}haracter{BERT}: Reconciling {ELM}o and {BERT} for Word-Level Open-Vocabulary Representations From Characters",
    author = "El Boukkouri, Hicham  and
      Ferret, Olivier  and
      Lavergne, Thomas  and
      Noji, Hiroshi  and
      Zweigenbaum, Pierre  and
      Tsujii, Jun{'}ichi",
    booktitle = "Proceedings of the 28th International Conference on Computational Linguistics",
    month = dec,
    year = "2020",
    address = "Barcelona, Spain (Online)",
    publisher = "International Committee on Computational Linguistics",
    url = "https://www.aclweb.org/anthology/2020.coling-main.609",
    doi = "10.18653/v1/2020.coling-main.609",
    pages = "6903--6915",
    abstract = "Due to the compelling improvements brought by BERT, many recent representation models adopted the Transformer architecture as their main building block, consequently inheriting the wordpiece tokenization system despite it not being intrinsically linked to the notion of Transformers. While this system is thought to achieve a good balance between the flexibility of characters and the efficiency of full words, using predefined wordpiece vocabularies from the general domain is not always suitable, especially when building models for specialized domains (e.g., the medical domain). Moreover, adopting a wordpiece tokenization shifts the focus from the word level to the subword level, making the models conceptually more complex and arguably less convenient in practice. For these reasons, we propose CharacterBERT, a new variant of BERT that drops the wordpiece system altogether and uses a Character-CNN module instead to represent entire words by consulting their characters. We show that this new model improves the performance of BERT on a variety of medical domain tasks while at the same time producing robust, word-level, and open-vocabulary representations.",
}