A Beginner's Guide to BERT: Understanding and Implementing Bidirectional Encoder Representations from Transformers
Published: 2023-07-07 14:49:57

Hey, Dev Community! In this post, we will explore BERT (Bidirectional Encoder Representations from Transformers), an influential AI model that has revolutionized natural language processing. Join me as we delve into the inner workings of BERT, its applications, advantages, and how it has transformed various NLP tasks.


!["Image description"](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/sbbdwo8mdnkdowmhax55.png)

## Article Summary

- Understanding BERT
- Need of BERT
- Pre-training and Fine-tuning
- Unleashing Contextual Word Representations
- Usage of BERT
- Applications of BERT
- Advantages of BERT
- Limitations and Challenges
- Conclusion

## Understanding BERT
BERT (Bidirectional Encoder Representations from Transformers) utilizes the Transformer model to capture bidirectional context in text. Unlike traditional models that read text in a left-to-right or right-to-left manner, BERT reads the entire input text bidirectionally, enabling it to capture the meaning of words based on their surrounding context.

## Need of BERT
One of the biggest challenges in NLP is the lack of enough training data. Overall there is enormous amount of text data available, but if we want to create task-specific datasets, we need to split that pile into the very many diverse fields. And when we do this, we end up with only a few thousand or a few hundred thousand human-labeled training examples. Unfortunately, in order to perform well, deep learning based NLP models require much larger amounts of data — they see major improvements when trained on millions, or billions, of annotated training examples.

## Pre-Training and Fine-Tuning
To help bridge the gap in data, researchers have developed various techniques for training general purpose language representation models using the enormous piles of unannotated text on the web, this is known as pre-training. These general purpose pre-trained models can then be fine-tuned on smaller task-specific datasets, e.g., when working with problems like question answering and sentiment analysis. This approach results in great accuracy improvements compared to training on the smaller task-specific datasets from scratch. BERT is a recent addition to these techniques for NLP pre-training; it caused a stir in the deep learning community because it presented state-of-the-art results in a wide variety of NLP tasks, like question answering.

The best part about BERT is that it can be download and used for free — we can either use the BERT models to extract high quality language features from our text data, or we can fine-tune these models on a specific task, like sentiment analysis and question answering, with our own data to produce state-of-the-art predictions.

## Unleashing Contextual Word Representations
BERT relies on a Transformer (the attention mechanism that learns contextual relationships between words in a text). A basic Transformer consists of an encoder to read the text input and a decoder to produce a prediction for the task. Since BERT’s goal is to generate a language representation model, it only needs the encoder part. The input to the encoder for BERT is a sequence of tokens, which are first converted into vectors and then processed in the neural network. But before processing can start, BERT needs the input to be massaged and decorated with some extra metadata:

1. **Token embeddings**: A [CLS] token is added to the input word tokens at the beginning of the first sentence and a [SEP] token is inserted at the end of each sentence.
2. **Segment embeddings**: A marker indicating Sentence A or Sentence B is added to each token. This allows the encoder to distinguish between sentences.
3. **Positional embeddings**: A positional embedding is added to each token to indicate its position in the sentence.

!["Image description"](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/h0x7g9wq42hgt1t7gp2u.png)

#### Architecture
There are four types of pre-trained versions of BERT depending on the scale of the model architecture:

_BERT-Base_: 12-layer, 768-hidden-nodes, 12-attention-heads, 110M parameters
_BERT-Large_: 24-layer, 1024-hidden-nodes, 16-attention-heads, 340M parameters

## Practical Usage of BERT
```
import tensorflow as tf
import tensorflow_hub as hub
from tensorflow import keras
import tensorflow_text as text

# Load the BERT preprocessing layer
bert_preprocess = hub.KerasLayer("https://tfhub.dev/tensorflow/bert_en_uncased_preprocess/3")

# Load the BERT encoder layer
bert_encoder = hub.KerasLayer("https://tfhub.dev/tensorflow/bert_en_uncased_L-12_H-768_A-12/4")

# Define the input text
sample = ["I love this music",
          "They smell very bad",
          "Everyone is looking beautiful",
          "I hate this book"]

# Preprocess the input text
preprocessed_sample = bert_preprocess(sample)
 
# Generate BERT embeddings
bert_outputs = bert_encoder(preprocessed_sample)

# Perform sentiment analysis
inputs = bert_outputs['pooled_output']
outputs = keras.layers.Dense(1, activation='sigmoid')(inputs)
model = keras.Model(inputs=bert_outputs, outputs=outputs)

# Define the example sentiment analysis function
def prediction(review):
    score = model.predict(review)
    score = score[0]
    if score < 0.5:
        print("Negative")
    else:
        print("Positive")
    print(score)

# Perform sentiment analysis on the sample text
prediction(bert_outputs)
```
This code demonstrates how to use BERT for sentiment analysis. First, the BERT preprocessing and encoding layers are loaded. Then, the input text is preprocessed using the BERT preprocessing layer, and BERT embeddings are generated using the BERT encoder layer. The BERT embeddings are passed through a dense layer with sigmoid activation to obtain the sentiment analysis prediction.

The prediction function takes the BERT embeddings as input and performs sentiment analysis by predicting the sentiment score for each example. If the score is below 0.5, it is classified as "Negative," otherwise as "Positive."

This example showcases the usage of BERT for sentiment analysis, which can be a valuable addition to a developer's toolkit when working with natural language processing tasks.

## Applications of BERT

1. Sentiment Analysis: BERT can analyze the sentiment expressed in a piece of text, classifying it as positive, negative, or neutral.

2. Question-Answering: BERT can understand the context of a question and provide accurate answers by extracting relevant information from the given text.

3. Named Entity Recognition: BERT can identify and classify named entities such as people, organizations, locations, and more, in a given text.

4. Text Classification: BERT can classify text into different categories or labels, such as topic classification, intent classification, or document classification.

5. Text Summarization: BERT can generate concise summaries of longer texts by extracting the most important information and preserving the context.

6. Language Translation: BERT can be used in machine translation tasks, where it translates text from one language to another by capturing the context and semantics.

7. Information Extraction: BERT can extract structured information from unstructured text, such as extracting key facts, relationships, or events.

8. Text Similarity and Clustering: BERT can measure the similarity between two pieces of text or group similar texts together based on their semantic meaning.

9. Natural Language Understanding (NLU): BERT enhances NLU tasks by understanding the meaning and context of user queries, enabling more accurate and personalized responses.

10. Chatbots and Virtual Assistants: BERT can power chatbots and virtual assistants to have more intelligent and human-like conversations, providing accurate and context-aware responses.

The versatility of BERT allows it to be applied across a wide range of NLP tasks, making it a valuable tool for developers in various domains.

## Advantages of BERT

- Captures Contextual Information: BERT considers the surrounding words to capture rich contextual information, enhancing the understanding of word meanings.
- Handles Long-Range Dependencies: BERT effectively captures relationships between words that are far apart in a sentence, handling long-range dependencies.
- Enables Transfer Learning: Pre-training on unlabeled data allows BERT to learn general language representations and fine-tune on specific tasks, enabling transfer learning.
- Supports Multiple Languages: BERT is trained on multilingual corpora, making it applicable to different languages.
- Generates Accurate Predictions: BERT's pre-training on extensive data leads to accurate predictions in various NLP tasks.


## Limitations and Challenges
- Computational Requirements: BERT is a resource-intensive model, demanding significant computational resources for training and inference.
- Fine-Tuning on Specific Tasks: Fine-tuning BERT requires task-specific labeled data, which can be time-consuming and costly.
- Domain Adaptation: BERT's performance may vary across different domains, necessitating additional efforts for domain adaptation.
- Handling Out-of-Vocabulary Words: BERT has a fixed vocabulary size, making it challenging to handle out-of-vocabulary words.
- Potential Bias and Ethical Considerations: BERT can inherit biases from the training data, leading to biased predictions. Ethical considerations should be taken into account.

## Conclusion
BERT has had a profound impact on natural language processing, demonstrating its capabilities in various NLP tasks. By understanding BERT's architecture, pre-training, fine-tuning, and applications, developers can leverage its power to enhance their NLP projects. BERT's ability to capture contextual information and generate accurate predictions has opened up new possibilities in language understanding.

## References and Further Readings
- [Original Paper](https://arxiv.org/abs/1810.04805)
- [Google-Research GitHub Repo](https://github.com/google-research/bert)
- [Blog Post by Google AI](https://ai.googleblog.com/2018/11/open-sourcing-bert-state-of-art-pre.html)
- [Colab Notebook: Predicting Movie Review Sentiment with BERT on TF Hub](https://colab.research.google.com/github/google-research/bert/blob/master/predicting_movie_reviews_with_bert_on_tf_hub.ipynb#scrollTo=xiYrZKaHwV81)
- [Using BERT for Binary Text Classification in PyTorch](https://medium.com/swlh/a-simple-guide-on-using-bert-for-text-classification-bbf041ac8d04)