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LSTM/BERT-CRF Model for Named Entity Recognition (or Sequence Labeling)

This repository implements an LSTM-CRF model for named entity recognition. The model is same as the one by Lample et al., (2016) except we do not have the last tanh layer after the BiLSTM. We achieve the SOTA performance on both CoNLL-2003 and OntoNotes 5.0 English datasets (check our benchmark with Glove and ELMo, other and benchmark results with fine-tuning BERT).

Announcements

We implemented distributed training for faster training
We implemented a Faster CRF module which allows O(log N) inference and back-tracking!
Benchmark results by fine-tuning BERT/Roberta**

Model	Dataset	Precision	Recall	F1
BERT-base-cased + CRF (this repo)	CONLL-2003	91.69	92.05	91.87
Roberta-base + CRF (this repo)	CoNLL-2003	91.88	93.01	92.44
BERT-base-cased + CRF (this repo)	OntoNotes 5	89.57	89.45	89.51
Roberta-base + CRF (this repo)	OntoNotes 5	90.12	91.25	90.68

More details

Requirements

Python >= 3.6 and PyTorch >= 1.6.0 (tested)
pip install transformers
pip install datasets
pip install accelerate (optional for distributed training)
pip install seqeval (optional, only used in evaluation while in distributed training)

In the documentation below, we present two ways for users to run the code:

Run the model via (Fine-tuning) BERT/Roberta/etc in Transformers package.
Run the model with simply word embeddings (and static ELMo/BERT representations loaded from external vectors).

Our default argument setup refers to the first one 1.

Usage with Fine-Tuning BERT/Roberta (,etc) models in HuggingFace

Simply replace the embedder_type argument with the model in HuggingFace. For example, if we are using roberta-large, we just need to change the embedder type as roberta-large.
```
python transformers_trainer.py --device=cuda:0 --dataset=YourData --model_folder=saved_models --embedder_type=roberta-base
```
Distributed Training (If necessary)
1. We use huggingface accelerate package to enable distributed training. After you set the proper configuration of your distributed environment, by accelerate config, you can easily run the following command for distributed training
```
accelerate launch transformers_trainer_ddp.py --batch_size=30 {YOUR_OTHER_ARGUMENTS}
```
Note that, this batch size is actually batch_size per GPU device.
(Optional) Using other models in HuggingFace.
1. Run the main file with modified argument embedder_type:
```
python trainer.py --embedder_type=bert-large-cased
```
  The default value for embedder_type is roberta-base. Changing the name to something like bert-base-cased or roberta-large, we directly load the model from huggingface. Note: if you use other models, remember to replace the tokenization mechanism in config/utils.py.
  
  Our default tokenizer is assumed to be fast_tokenizer. If your tokenizer does not support fast mode, try set use_fast=False:
```
tokenizer = AutoTokenizer.from_pretrained(conf.embedder_type, add_prefix_space=True, use_fast=False)
```
2. Finally, if you would like to know more about the details, read more details below:
  - Tokenization: For BERT, we use the first wordpice to represent a complete word. Check config/transformers_util.py
  - Embedder: We show how to embed the input tokens to make word representation. Check model/embedder/transformers_embedder.py
3. Using BERT/Roberta as contextualized word embeddings (Static, Feature-based Approach) Simply go to model/transformers_embedder.py and uncomment the following:
```
self.model.requires_grad = False
```

Other Usages

Using Word embedding or external contextualized embedding (ELMo/BERT) can be found in here.

Training with your own data.

Create a folder YourData under the data directory.
Put the train.txt, dev.txt and test.txt files (make sure the format is compatible, i.e. the first column is words and the last column are tags) under this directory. If you have a different format, simply modify the reader in config/reader.py.
Change the dataset argument to YourData when you run trainer.py.