TaCL: Improving BERT Pre-training with Token-aware Contrastive Learning

Authors: Yixuan Su, Fangyu Liu, Zaiqiao Meng, Lei Shu, Ehsan Shareghi, and Nigel Collier

Code of our paper: TaCL: Improving BERT Pre-training with Token-aware Contrastive Learning

Introduction:

Masked language models (MLMs) such as BERT and RoBERTa have revolutionized the field of Natural Language Understanding in the past few years. However, existing pre-trained MLMs often output an anisotropic distribution of token representations that occupies a narrow subset of the entire representation space. Such token representations are not ideal, especially for tasks that demand discriminative semantic meanings of distinct tokens. In this work, we propose TaCL (Token-aware Contrastive Learning), a novel continual pre-training approach that encourages BERT to learn an isotropic and discriminative distribution of token representations. TaCL is fully unsupervised and requires no additional data. We extensively test our approach on a wide range of English and Chinese benchmarks. The results show that TaCL brings consistent and notable improvements over the original BERT model. Furthermore, we conduct detailed analysis to reveal the merits and inner-workings of our approach

Main Results:

We show the comparison between TaCL (base version) and the original BERT (base version).

(1) English benchmark results on SQuAD (Rajpurkar et al., 2018) (dev set) and GLUE (Wang et al., 2019) average score.

(2) Chinese benchmark results (test set F1) on four NER tasks (MSRA, OntoNotes, Resume, and Weibo) and three Chinese word segmentation (CWS) tasks (PKU, CityU, and AS).

Huggingface Models:

Example Usage:

import torch
# initialize model
from transformers import AutoModel, AutoTokenizer
model_name = 'cambridgeltl/tacl-bert-base-uncased'
model = AutoModel.from_pretrained(model_name)
tokenizer = AutoTokenizer.from_pretrained(model_name)
# create input ids
text = '[CLS] clbert is awesome. [SEP]'
tokenized_token_list = tokenizer.tokenize(text)
input_ids = torch.LongTensor(tokenizer.convert_tokens_to_ids(tokenized_token_list)).view(1, -1)
# compute hidden states
representation = model(input_ids).last_hidden_state # [1, seqlen, embed_dim]

Tutorial (in Chinese language) on how to use Chinese TaCL BERT to performance Name Entity Recognition and Chinese word segmentation:

Tutorial link

Tutorial on how to reproduce the results in our paper:

1. Environment Setup:

python version: 3.8
pip3 install -r requirements.txt

2. Train TaCL:

(1) Prepare pre-training data:

Please refer to details provided in ./pretraining_data directory.

(2) Train the model:

Please refer to details provided in ./pretraining directory.

3. Experiments on English Benchmarks:

Please refer to details provided in ./english_benchmark directory.

4. Experiments on Chinese Benchmarks:

(1) Chinese Benchmark Data Preparation:

chmod +x ./download_benchmark_data.sh
./download_benchmark_data.sh

(2) Fine-tuning and Inference:

Please refer to details provided in ./chinese_benchmark directory.

5. Replicate Our Analysis Results:

We provide all essential code to replicate the results (the images below) provided in our analysis section. The related codes and instructions are located in ./analysis directory. Have fun!

Citation:

If you find our paper and resources useful, please kindly cite our paper:

@misc{su2021tacl,
      title={TaCL: Improving BERT Pre-training with Token-aware Contrastive Learning}, 
      author={Yixuan Su and Fangyu Liu and Zaiqiao Meng and Lei Shu and Ehsan Shareghi and Nigel Collier},
      year={2021},
      eprint={2111.04198},
      archivePrefix={arXiv},
      primaryClass={cs.CL}
}

Contact

If you have any questions, feel free to contact me via ([email protected]).