ELMER

This repository contains code and checkpoints for ELMER:

ELMER: A Non-Autoregressive Pre-trained Language Model for Efficient and Effective Text Generation

Junyi Li, Tianyi Tang, Wayne Xin Zhao, Jian-Yun Nie, Ji-Rong Wen (EMNLP 2022)

Introduction

To explicitly learn the bi-directional token dependency, we propose ELMER: an Efficient and Effective PLM for NAR text generation, which generates tokens at different layers by leveraging the early exit technique.

The architecture of ELMER is a variant of the standard Transformer encoder-decoder and poses three technical contributions:

1. For decoder, we replace the original masked multi-head attention with bi-directional multi-head attention akin to the encoder. Therefore, ELMER dynamically adjusts the output length by emitting an end token "[EOS]" at any position.
2. Leveraging early exit, ELMER injects "off-ramps" at each decoder layer, which make predictions with intermediate hidden states. If ELMER exits at the $l$-th layer, we copy the $l$-th hidden states to the subsequent layers.
3. ELMER utilizes a novel pre-training objective, layer permutation language modeling (LPLM), to pre-train on the large-scale corpus. LPLM permutes the exit layer for each token from 1 to the maximum layer $L$.

Pre-trained Models

We provide the checkpoint for ELMER-base, which was pre-trained on 16GB English corpus, i.e., BookCorpus and Wikipedia.

• ELMER-base: 6 layers encoder, 6 layers decoder, 12 attention heads, and 768 hidden dimensions.

The checkpoint can be directly used with Hugging Face Transformers. In the future, we will integrate ELMER into Hugging Face and TextBox libraries for easy-to-use.

Requirements

To install requirements

bash install.sh

How to use

The pre-training code can be found here, and the fine-tuning code can be found here.

To fine-tune ELMER, please copy the file modeling_bart.py from the fine-tune directory to the BART directory in Transformers, such as ~/miniconda3/envs/[env_name]/lib/python3.7/site-packages/transformers/models/bart.

from transformers import BartTokenizer as ElmerTokenizer
from transformers import BartForConditionalGeneration as ElmerForConditionalGeneration

# pretrained_model/elmer-base is the saved directory for ELMER checkpoints
tokenizer = ElmerTokenizer.from_pretrained("pretrained_model/elmer-base")
model = ElmerForConditionalGeneration.from_pretrained("pretrained_model/elmer-base")

#--------------------------------
# do training for many many steps
#--------------------------------

For example, we'd like to fine-tune ELMER on XSUM dataset:

python train.py --dataset=XSUM --model=ELMER-XSUM --data-dir=[DATASET_DIR] \
       --pretrained_model_dir=[ELMER_BASE_DIR] --saved_dir=[FINE_TUNED_MODEL_DIR] --log-dir=[LOG_DIR] \
       --start_epoch=0 --epochs=100 --train_batch_size=32 --eval_batch_size=32 --optimizer=adam --lr=2e-5

These hyper-parameters can be also defined in config.yaml file.

Evaluation

To evaluate the generated texts, the BLEU, METEOR, and Distinct metrics can be computed using our provided scripts in pyeval package. For the ROUGE metric, please install the files2rouge package and compute it.

Contact

If you have any problems, raise an issue or contact lijunyi@ruc.edu.cn.

Citation

@article{lijunyi2022elmer,
  title={ELMER: A Non-Autoregressive Pre-trained Language Model for Efficient and Effective Text Generation},
  author={Li, Junyi and Tang, Tianyi and Zhao, Wayne Xin and Nie, Jian-Yun and Wen, Ji-Rong},
  booktitle={EMNLP 2022},
  year={2022}
}