Comparatively Fine-Tuning BERT

Introduction

The state-of-the-art pretrained language model BERT (Bidirectional Encoder Representations from Transformers) has achieved remarkable results in many natural language understanding tasks. In general, the industry-wide adoption of transformer architectures (BERT, XLNet, etc.) marked a sharp deviation from the conventional encoder-decoder architectures in sequence-to-sequence tasks such as machine translation. Following this, many parties are increasingly utilizing the pretrained weights of these language representation models, and fine-tuning them to suit whatever text-related task they are trying to solve.

Rooting from the observations above, this repository builds on top of papers like How to Fine-Tune BERT for Text Classification? by Chi Sun et al. and To Tune or Not To Tune? How About the Best of Both Worlds? by Ran Wang et al., and specifically focuses on minimizing the network depth of a BERT model to an extent where the accuracies for text classification tasks are still considerably high, yet the model size isn't as catastrophic as the advertised, full 12-layer architecture.

We hope that this work could help other people reduce their training time, fit their models in modest GPU and storage resources, and help them to further experiment with BERT. One key idea is to realize that sometimes we don't need the full architecture of a state-of-the-art model, but only parts of it, to achieve a sufficiently high performance on some given task. This idea is even more relevant when incorporating the full architecture is costly, and this is definitely the case with BERT and other similar transformer-based models.

Please note that the performance measures and any accompanying findings have been solely formulated on the classic IMDB dataset, and hence within a binary sentiment classification framework. We are planning to include more datasets in the near future for a more general & qualitative comparison.

Setting Up

1. Clone this repository: git clone https://github.com/uzaymacar/comparatively-finetuning-bert.git
2. Install PyTorch (installation)
3. Install Pytorch-Transformers: pip install pytorch_transformers
4. Download the IMDB dataset: bash download_imdb_dataset.sh

Important Implementation Notes

• For fairness of comparison, we are using the following components:

  • A batch size of 32 is used for models with number of transformer blocks lesser or equal to 4, a batch size of 16 is used for models with number of transformer blocks 5 or 6, and finally a batch size of 8 is used for the complete BERT model. This inverse relationship between the batch size hyperparameter and the complexity (hence size) of the model was adapted in order to fit the training processes into GPU.
  • Adam optimizer that utilizes the same grouped parameters with the same arguments across all finetuned & baseline model trainings. In particular, we have chosen to fine-tune BERT layers with a lower learning rate of 3e-5, and a fixed weight decay of 0.01 (for most parameters), and to optimize all other custom layers on top with a conventional learning rate of 1e-3 without no decay. As suggested in the paper, such a lower learning rate for BERT parameters is preferred over an aggressive one to overcome the common problem of catastrophic forgetting (the forgetting of pretrained knowledge) in transfer learning applications.
  • GPU choice of Tesla K80 with a maximum number of 10 epochs.
  • If applicable, a single layer, bidirectional configuration for LSTM layers with a hidden size of 128.

• All BERT models are pretrained from the bert-base-cased model weights. For more information about this and other models, check here.

• The hidden states extracted from finetuned models, whether directly acquired from the pretrained BERT model or acquired from the additional recurrent (LSTM) layers, undergo a dropout of default rate 0.20.

• The pytorch-transformers implementation of BERT is adapted in this repository to output 4 different tensors:

  1. Sequence Output, (B, P, H): The final hidden states outputted from the encoder in the last BERT layer
  2. Pooled Output, (B, H): The final hidden vector associated with the first input token ([CLS]) outputted from the encoder in the last BERT layer, and underwent a linear layer with tanh() activation
  3. All Hidden States, (([K+1] x (B, P, H))): Hidden states outputted from the encoders of all the BERT layers
  4. Attention Output, (([K] x (B, N, P, P))): Multi-head attention weights extracted from all the BERT layers

• It should be noted that the final hidden states, and hence the last element of the all hidden states tuple is equivalent to sequence output.

• It should also be noted that although the model class gives the option to not utilize the pooler layers with pretrained weights and reinitialize them instead, doing so yielded surprisingly low accuracies.

• Finally, we abandon configurations with concatenations and bottom-down weight assignment for models as the number of transformers blocks increase, because they were both memory intensive and yielded lower accuracies compared to other configurations.

Comparison & Analysis of Results

The saved model files (.pt) of all models mentioned below can be located in a Google Drive folder here with the corresponding model path provided. Make sure to use the latest uploaded version for each model for reproducing accuracies provided here. Check PyTorch's documentation to load them.

Baseline Models

Finetuned Models

Utilizing 1 BERT Transformer Block

Utilizing 2 BERT Transformer Blocks

Utilizing 3 BERT Transformer Blocks

Utilizing 4 BERT Transformer Blocks

Utilizing 5 BERT Transformer Blocks

Utilizing 6 BERT Transformer Blocks

Complete (Utilizing 12 Transformer Blocks) BERT Model

Comments

• The maximum accuracy we have reached with the complete model agree with this issue. Moreover, the performance is relatively comparable to the transformer-based model performances mentioned here and here.
• It should be mentioned that a more intensive hyperparameter search and a more standard batch size might increase the performance of the complete BERT model. Moreover, the highest accuracy, 95.80, achieved by a BERT model in IMDB dataset did so by utilizing the large version of the BERT model (24 transformer blocks instead of 12, a hidden size of 1024 instead of 768, and 16 attention heads instead of 12) alongside unsupervised data augmentation. Read more about this here.
• Our experimentation with bidirectional LSTMs on top of BERT agree with the findings of Research of LSTM Additions on Top of SQuAD BERT Hidden Transform Layers by Adam Thorne et al. Hence, we agree with the following quote from the paper: "We saw worse performance and scores of roughly half a point lower than the baseline. Our team still believes in this strategy here, but perhaps there may be further optimizations in our hyperparameters or architecture. We believe the theory and merging of the two strategies do hold some value.
• Below is a graph visualizing the best performances we have achieved in this study, and the relationship between the number of transformer blocks and accuracy:

• As observed from the graph and the numbers reported above, the biggest increase in accuracy (delta) happens going from utilizing 2 transformer blocks to 3. To make this point clearer, the amount of increase observed here is greater than the amount increase observed while going from 6 transformer blocks to 12.

Visualization of Attention

Many researchers have argued, with evidence, that attention weights don't provide consistent explanations to model predictions. Moreover, several studies showed a low correlation between model-predicted visual attention and human attention. In this repository, however, we still investigated attention patterns as attention is a key component of transformer-based models.

For a pretrained BERT model, attention weights are likely to represent the complex linguistic dependencies between tokens and hence are not very interpretable. For a finetuned BERT model on binary sentiment classification, however, the attention weights should be more interpretable as they are likely to reveal tokens with certain sentiment attributes (positive or negative in this case).

There exists several approaches to compute attention weights in transformer-based models. One conventional approach is to extract the attention weights based on [CLS] token from the last layer, average each token weight across all attention heads, and finally normalize weights across all tokens so that each individual weight is a value between 0.0 (very low attention) and 1.0 (very high attention). The get_attention_average_last_layer() function inside utils/model_utils.py corresponds to this approach. Within the same file, you can find other approaches for extracting attention weights.

Running visualize_attention.py will display highlighted test movie reviews (examples). You can play around with constants LAYER_ID, HEAD_ID, and TOKEN_ID to visualize attentions located elsewhere in the BERT model. You can investigate get_normalized_attention() function in utils/model_utils.py to see all possible configurations. Note that you will need to have a ready-to-go saved model file (.pt) to visualize attentions.

The highlight colors are based on the following criteria:

Attention Weights (Score)	Description	Highlight Color
0.00 - 0.20	Very Low Attention	N/A
0.20 - 0.40	Low Attention
0.40 - 0.60	Medium Attention
0.60 - 0.80	High Attention
0.80 - 1.00	Very High Attention

The implementation of the color codes is in utils/visualization_utils.py and you can change it accordingly. We went with the Python package Tkinter for displaying attention scores, as opposed to JavaScript or a graphics library, for simplicity. The below visualizations are extracted from a BERT model with all 12 transformer blocks and 12 attention heads, but smaller models also have the potential to display meaningful insights.

The visualizations we have represented here should not be taken for granted, but we propose that they could be used alongside other conventional metrics to assess model performance.

Resources

• How to Fine-Tune BERT for Text Classification by Chi Sun, Xipeng Qiu, Yige Xu, and Xuanjing Huang. (arXiv)
• To Tune or Not To Tune? How About the Best of Both Worlds? by Ran Wang, Haibo Su, Chunye Wang, Kailin Ji, and Jupeng Ding (arXiv)
• Research of LSTM Additions on Top of SQuAD BERT Hidden Transform Layers by Adam Thorne, Zac Farnsworth, and Oscar Matus (web.stanford)
• PyTorch Transformers
• BertViz