airaria/TextBrewer

English | 中文说明

TextBrewer is a PyTorch-based model distillation toolkit for natural language processing. It includes various distillation techniques from both NLP and CV field and provides an easy-to-use distillation framework, which allows users to quickly experiment with the state-of-the-art distillation methods to compress the model with a relatively small sacrifice in the performance, increasing the inference speed and reducing the memory usage.

Check our paper through ACL Anthology or arXiv pre-print.

Full Documentation

News

Dec 17, 2021

• We have released a model pruning toolkit TextPruner. Check https://github.com/airaria/TextPruner

Oct 24, 2021

• We propose the first pre-trained language model that specifically focusing on Chinese minority languages. Check：https://github.com/ymcui/Chinese-Minority-PLM

Jul 8, 2021

• New examples with Transformers 4
  • The current examples (exmaples/) have been written with old versions of Transformers and they may cause some confusions and bugs. We rewrite the examples with Transformers 4 in Jupyter Notebooks, which are easy to follow and learn.
  • The new examples can be found at examples/notebook_examples. See Examples for details.

Mar 1, 2021

• BERT-EMD and custom distiller

  • We added an experiment with BERT-EMD in the MNLI exmaple. BERT-EMD allows each intermediate student layer to learn from any intermediate teacher layers adaptively, based on optimizing Earth Mover’s Distance. So there is no need to specify the matching scheme.
  • We have written a new EMDDistiller to perform BERT-EMD. It demonstrates how to write a custom distiller.

• updated MNLI example

  • We removed the pretrained_pytorch_bert and used transformers library instead in all the MNLI examples.

Table of Contents

Introduction

Textbrewer is designed for the knowledge distillation of NLP models. It provides various distillation methods and offers a distillation framework for quickly setting up experiments.

The main features of TextBrewer are:

• Wide-support: it supports various model architectures (especially transformer-based models)
• Flexibility: design your own distillation scheme by combining different techniques; it also supports user-defined loss functions, modules, etc.
• Easy-to-use: users don't need to modify the model architectures
• Built for NLP: it is suitable for a wide variety of NLP tasks: text classification, machine reading comprehension, sequence labeling, ...

TextBrewer currently is shipped with the following distillation techniques:

• Mixed soft-label and hard-label training
• Dynamic loss weight adjustment and temperature adjustment
• Various distillation loss functions: hidden states MSE, attention-matrix-based loss, neuron selectivity transfer, ...
• Freely adding intermediate features matching losses
• Multi-teacher distillation
• ...

TextBrewer includes:

1. Distillers: the cores of distillation. Different distillers perform different distillation modes. There are GeneralDistiller, MultiTeacherDistiller, BasicTrainer, etc.
2. Configurations and presets: Configuration classes for training and distillation, and predefined distillation loss functions and strategies.
3. Utilities: auxiliary tools such as model parameters analysis.

To start distillation, users need to provide

1. the models (the trained teacher model and the un-trained student model)
2. datasets and experiment configurations

TextBrewer has achieved impressive results on several typical NLP tasks. See Experiments.

See Full Documentation for detailed usages.

Architecture

Installation

• Requirements

  • Python >= 3.6
  • PyTorch >= 1.1.0
  • TensorboardX or Tensorboard
  • NumPy
  • tqdm
  • Transformers >= 2.0 (optional, used by some examples)
  • Apex == 0.1.0 (optional, mixed precision training)

• Install from PyPI

  pip install textbrewer

• Install from the Github source

  git clone https://github.com/airaria/TextBrewer.git
  pip install ./textbrewer

Workflow

• Stage 1: Preparation:

  1. Train the teacher model
  2. Define and initialize the student model
  3. Construct a dataloader, an optimizer, and a learning rate scheduler

• Stage 2: Distillation with TextBrewer:

  1. Construct a TraningConfig and a DistillationConfig, initialize a distiller
  2. Define an adaptor and a callback. The adaptor is used for adaptation of model inputs and outputs. The callback is called by the distiller during training
  3. Call the train method of the distiller

Quickstart

Here we show the usage of TextBrewer by distilling BERT-base to a 3-layer BERT.

Before distillation, we assume users have provided:

• A trained teacher model teacher_model (BERT-base) and a to-be-trained student model student_model (3-layer BERT).
• a dataloader of the dataset, an optimizer and a learning rate builder or class scheduler_class and its args dict scheduler_dict.

Distill with TextBrewer:

import textbrewer
from textbrewer import GeneralDistiller
from textbrewer import TrainingConfig, DistillationConfig

# Show the statistics of model parameters
print("\nteacher_model's parametrers:")
result, _ = textbrewer.utils.display_parameters(teacher_model,max_level=3)
print (result)

print("student_model's parametrers:")
result, _ = textbrewer.utils.display_parameters(student_model,max_level=3)
print (result)

# Define an adaptor for interpreting the model inputs and outputs
def simple_adaptor(batch, model_outputs):
      # The second and third elements of model outputs are the logits and hidden states
    return {'logits': model_outputs[1],
            'hidden': model_outputs[2]}

# Training configuration 
train_config = TrainingConfig()
# Distillation configuration
# Matching different layers of the student and the teacher
distill_config = DistillationConfig(
    intermediate_matches=[    
     {'layer_T':0, 'layer_S':0, 'feature':'hidden', 'loss': 'hidden_mse','weight' : 1},
     {'layer_T':8, 'layer_S':2, 'feature':'hidden', 'loss': 'hidden_mse','weight' : 1}])

# Build distiller
distiller = GeneralDistiller(
    train_config=train_config, distill_config = distill_config,
    model_T = teacher_model, model_S = student_model, 
    adaptor_T = simple_adaptor, adaptor_S = simple_adaptor)

# Start!
with distiller:
    distiller.train(optimizer, dataloader, num_epochs=1, scheduler_class=scheduler_class, scheduler_args = scheduler_args, callback=None)

Examples

• Notebook examples with Transformers 4

  • examples/notebook_examples/sst2.ipynb (English): training and distilling BERT on SST-2, an English sentence classification task.
  • examples/notebook_examples/msra_ner.ipynb (Chinese): training and distilling BERT on MSRA NER, a Chinese sequence labeling task.
  • examples/notebook_examples/sqaudv1.1.ipynb (English): training and distilling BERT on SQuAD 1.1, an English MRC task.

• examples/random_token_example : a simple runnable toy example which demonstrates the usage of TextBrewer. This example performs distillation on the text classification task with random tokens as inputs.

• examples/cmrc2018_example (Chinese): distillation on CMRC 2018, a Chinese MRC task, using DRCD as data augmentation.

• examples/mnli_example (English): distillation on MNLI, an English sentence-pair classification task. This example also shows how to perform multi-teacher distillation.

• examples/conll2003_example (English): distillation on CoNLL-2003 English NER task, which is in form of sequence labeling.

• examples/msra_ner_example (Chinese): This example distills a Chinese-ELECTRA-base model on the MSRA NER task with distributed data-parallel training(single node, multi-GPU).

Experiments

We have performed distillation experiments on several typical English and Chinese NLP datasets. The setups and configurations are listed below.

Models

• For English tasks, the teacher model is BERT-base-cased.
• For Chinese tasks, the teacher models are RoBERTa-wwm-ext and Electra-base released by the Joint Laboratory of HIT and iFLYTEK Research.

We have tested different student models. To compare with public results, the student models are built with standard transformer blocks except for BiGRU which is a single-layer bidirectional GRU. The architectures are listed below. Note that the number of parameters includes the embedding layer but does not include the output layer of each specific task.

English models

Chinese models

• T6 architecture is the same as DistilBERT^[1], BERT₆-PKD^[2], and BERT-of-Theseus^[3].
• T4-tiny architecture is the same as TinyBERT^[4].
• T3 architecure is the same as BERT₃-PKD^[2].

Distillation Configurations

distill_config = DistillationConfig(temperature = 8, intermediate_matches = matches)
# Others arguments take the default values

matches are differnt for different models:

The definitions of matches are at examples/matches/matches.py.

We use GeneralDistiller in all the distillation experiments.

Training Configurations

• Learning rate is 1e-4 (unless otherwise specified).
• We train all the models for 30~60 epochs.

Results on English Datasets

We experiment on the following typical English datasets:

We list the public results from DistilBERT, BERT-PKD, BERT-of-Theseus, TinyBERT and our results below for comparison.

Public results:

Our results:

Note:

1. The equivalent model structures of public models are shown in the brackets after their names.
2. When distilling to T4-tiny, NewsQA is used for data augmentation on SQuAD and HotpotQA is used for data augmentation on CoNLL-2003.
3. When distilling to T12-nano, HotpotQA is used for data augmentation on CoNLL-2003.

Results on Chinese Datasets

We experiment on the following typical Chinese datasets:

The results are listed below.

Note:

1. Learning rate decay is not used in distillation on CMRC 2018 and DRCD.
2. CMRC 2018 and DRCD take each other as the augmentation dataset in the distillation.
3. The settings of training Electra-base teacher model can be found at Chinese-ELECTRA.
4. Electra-small student model is initialized with the pretrained weights.

Core Concepts

Configurations

• TrainingConfig: configuration related to general deep learning model training
• DistillationConfig: configuration related to distillation methods

Distillers

Distillers are in charge of conducting the actual experiments. The following distillers are available:

• BasicDistiller: single-teacher single-task distillation, provides basic distillation strategies.
• GeneralDistiller (Recommended): single-teacher single-task distillation, supports intermediate features matching. Recommended most of the time.
• MultiTeacherDistiller: multi-teacher distillation, which distills multiple teacher models (of the same task) into a single student model. This class doesn't support Intermediate features matching.
• MultiTaskDistiller: multi-task distillation, which distills multiple teacher models (of different tasks) into a single student.
• BasicTrainer: Supervised training a single model on a labeled dataset, not for distillation. It can be used to train a teacher model.

User-Defined Functions

In TextBrewer, there are two functions that should be implemented by users: callback and adaptor.

Callback

At each checkpoint, after saving the student model, the callback function will be called by the distiller. A callback can be used to evaluate the performance of the student model at each checkpoint.

Adaptor

It converts the model inputs and outputs to the specified format so that they could be recognized by the distiller, and distillation losses can be computed. At each training step, batch and model outputs will be passed to the adaptor; the adaptor re-organizes the data and returns a dictionary.

For more details, see the explanations in Full Documentation.

FAQ

Q: How to initialize the student model?

A: The student model could be randomly initialized (i.e., with no prior knowledge) or be initialized by pre-trained weights. For example, when distilling a BERT-base model to a 3-layer BERT, you could initialize the student model with RBT3 (for Chinese tasks) or the first three layers of BERT (for English tasks) to avoid cold start problem. We recommend that users use pre-trained student models whenever possible to fully take advantage of large-scale pre-training.

Q: How to set training hyperparameters for the distillation experiments？

A: Knowledge distillation usually requires more training epochs and larger learning rate than training on the labeled dataset. For example, training SQuAD on BERT-base usually takes 3 epochs with lr=3e-5; however, distillation takes 30~50 epochs with lr=1e-4. The conclusions are based on our experiments, and you are advised to try on your own data.

Q: My teacher model and student model take different inputs (they do not share vocabularies), so how can I distill?

A: You need to feed different batches to the teacher and the student. See the section Feed Different batches to Student and Teacher, Feed Cached Values in the full documentation.

Q: I have stored the logits from my teacher model. Can I use them in the distillation to save the forward pass time?

A: Yes, see the section Feed Different batches to Student and Teacher, Feed Cached Values in the full documentation.

Known Issues

• Multi-GPU training support is only available through DataParallel currently.
• Multi-label classification is not supported.

Citation

If you find TextBrewer is helpful, please cite our paper:

@InProceedings{textbrewer-acl2020-demo,
    title = "{T}ext{B}rewer: {A}n {O}pen-{S}ource {K}nowledge {D}istillation {T}oolkit for {N}atural {L}anguage {P}rocessing",
    author = "Yang, Ziqing and Cui, Yiming and Chen, Zhipeng and Che, Wanxiang and Liu, Ting and Wang, Shijin and Hu, Guoping",
    booktitle = "Proceedings of the 58th Annual Meeting of the Association for Computational Linguistics: System Demonstrations",
    year = "2020",
    publisher = "Association for Computational Linguistics",
    url = "https://www.aclweb.org/anthology/2020.acl-demos.2",
    pages = "9--16",
}

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