Self-attention Made Easy & How To Implement It In PyTorch

by | Jan 31, 2023 | Machine Learning, Natural Language Processing

Self-attention is the reason transformers are so successful at many NLP tasks. Learn how they work, the different types, and how to implement them with PyTorch in Python.

What is self-attention in deep learning?

Self-attention is a type of attention mechanism used in deep learning models, also known as the self-attention mechanism. It lets a model decide how important each part of an input sequence is, which makes it possible to find dependencies and connections in the data.

Self-attention is used extensively in deep learning architectures, especially in natural language processing (NLP). For example, tasks like machine translation, sentiment analysis, and question-answering depend significantly on self-attention.

In self-attention, a model calculates the attention weights between each element in the input sequence, allowing it to focus on the relevant factors for a given task. This mechanism works very well because it lets the model take into account long-term dependencies and relationships in the data, which improves performance on many jobs.

Self-attention looks for relationships in the data.

Self-attention looks for relationships in the data.

Meaning

Self-attention is a deep learning mechanism that lets a model focus on different parts of an input sequence by giving each part a weight to figure out how important it is for making a prediction.

The model uses this self-attention mechanism to decide which parts of the input to focus on dynamically. In addition, it allows it to handle input sequences of varying lengths and capture dependencies between elements in the series.

Transformer

The Transformer is an architecture for deep learning that uses mechanisms for self-attention to process sequential data like text. In the Transformer, self-attention is used to determine how much attention each part of the input sequence gets. This lets the model figure out each part’s importance and make predictions based on that.

The attention mechanism allows the Transformer to capture long-range dependencies in the input sequence and handle input of varying lengths. The Transformer has become one of the most popular architectures in natural language processing and has done state-of-the-art work on various tasks.

Self-attention example

An example of self-attention in deep learning is its use in machine translation. In this task, a model takes a source sentence in one language as input and produces a translated sentence in another.

Using self-attention, the model can focus on different parts of the source sentence, assigning weights to each piece to determine its importance in the translation.

For example, in a sentence like “I will go to the park with my friends,” the model may give more weight to the word “park” because it is an essential aspect of the sentence that needs to be translated correctly. The self-attention mechanism allows the model to make these dynamic, context-specific decisions, improving the accuracy of the translation.

Types of self-attention

There are several types of self-attention mechanisms used in deep learning, including:

  1. Dot-product attention: The attention scores are calculated as the dot product of the queries and keys. This type of self-attention is used in the Transformer architecture.
  2. Scaled dot-product attention is similar to dot-product attention, but the attention scores are divided by the square root of the number of dimensions of the queries and keys to ensure they are stable.
  3. Multi-head attention: Multiple attention heads capture different aspects of the input sequence. Each head calculates its own set of attention scores, and the results are concatenated and transformed to produce the final attention weights.
  4. Local attention: The attention mechanism is only used on several elements in the input sequence. This lets the model focus on dependencies close to the sequence’s beginning or end.
  5. Additive attention: The attention scores are calculated as a function of the similarity between the queries and keys rather than just their dot product.
  6. Cosine attention scores are calculated as the cosine similarity between the queries and keys.

These are some of the most commonly used self-attention mechanisms in deep learning. The choice of a self-attention mechanism depends on the specific task and the desired properties of the model.

Self-attention vs attention

Self-attention and attention are similar mechanisms in deep learning, but there is a critical difference between the two.

Attention refers to a mechanism in which a model calculates attention scores between different parts of an input and another part of the input or external memory. For example, in machine translation, the attention mechanism calculates attention scores between the source sentence and the target sentence, allowing the model to weigh the importance of each part of the source sentence in the target translation.

On the other hand, self-attention is a mechanism by which the model calculates attention scores between different parts of the input sequence without using external memory. Self-attention lets the model figure out how important each part of the series is, determine how the parts depend on each other and make predictions based on that.

In short, attention is a mechanism in which a model calculates attention scores between different parts of an input and another part of the input or external memory. On the other hand, self-attention is a mechanism in which the model only calculates attention scores between different parts of the input sequence.

Self-attention in NLP

Self-attention is crucial in many deep learning models for natural language processing (NLP).

For example, in NLP, figuring out how important each part of a sequence is is especially helpful because it lets a model better understand how words in a sentence depend on each other and work together.

The Transformer architecture is one of the most successful ways that self-attention has been used in NLP. It has been used for machine translation, sentiment analysis, and question-answering tasks.

For example, self-attention is used in the Transformer to figure out the attention weights between each word in the input sequence. This lets the model focus on essential words for a particular task.

Another example of self-attention in NLP is the self-attention mechanism used in transformer-based language models such as BERT and GPT-3. These models use self-attention to figure out how words in a sequence relate to each other and make representations that can be used for tasks like figuring out how someone feels or answering a question.

Self-attention is a handy tool in NLP. It lets models take into account long-term dependencies and relationships between words, which improves performance on many NLP tasks.

Self-attention tutorial

Here is a high-level overview of how to implement self-attention in deep learning:

  1. Prepare your input data: The first step is to prepare your input data, usually a sequence of data such as text or a time series.
  2. Calculate attention scores: The next step is calculating the attention scores between each element in the input sequence. This is done by applying a multi-layer feedforward neural network to each aspect, generating a set of attention scores representing each element’s importance in the series.
  3. Apply attention mechanism: Using the attention scores, the attention mechanism can be applied to the input sequence. This is done by weighting each element in the series by its attention score, producing a weighted representation of the input sequence.
  4. Pass the weighted representation through the model: The weighted term of the input sequence is then passed through the rest of the model, typically a series of fully connected layers, to make predictions.
  5. Train the model: Finally, the model is trained using a supervised learning algorithm, like cross-entropy loss, to minimise the prediction error.

This is a high-level overview of the self-attention mechanism. Reading related research papers and tutorials is recommended for a more in-depth understanding and implementation details.

Implement self-attention in PyTorch Example

Here’s an example of how to implement self-attention in PyTorch:

import torch
import torch.nn as nn
import torch.nn.functional as F

class SelfAttention(nn.Module):
    def __init__(self, input_dim):
        super(SelfAttention, self).__init__()
        self.input_dim = input_dim
        self.query = nn.Linear(input_dim, input_dim)
        self.key = nn.Linear(input_dim, input_dim)
        self.value = nn.Linear(input_dim, input_dim)
        self.softmax = nn.Softmax(dim=2)
        
    def forward(self, x):
        queries = self.query(x)
        keys = self.key(x)
        values = self.value(x)
        scores = torch.bmm(queries, keys.transpose(1, 2)) / (self.input_dim ** 0.5)
        attention = self.softmax(scores)
        weighted = torch.bmm(attention, values)
        return weighted

In this example, the SelfAttention module takes an input tensor x with shape (batch_size, seq_length, input_dim) and returns a weighted representation of the input sequence with the same form.

The attention mechanism is implemented using dot-product attention, where the query, key, and value vectors are learned through linear transformations of the input sequence.

The attention scores are then calculated as the dot product of the queries and keys, and the attention is applied by multiplying the values by the attention scores. The result is a weighted representation of the input sequence that considers each element’s importance.

Conclusion

In conclusion, self-attention is a powerful part of deep learning that lets a model figure out how vital each sequence part is. This lets the model better understand dependencies and relationships in the data.

Self-attention has been used in various architectures, such as the Transformer, and has shown great success in many tasks, including natural language processing and computer vision.

There are many types of attention mechanisms, each with its advantages and disadvantages, and the choice of the mechanism depends on the specific task and desired properties of the model.

Overall, self-attention is a promising technique that has the potential to improve the performance of many deep-learning models.

About the Author

Neri Van Otten

Neri Van Otten

Neri Van Otten is the founder of Spot Intelligence, a machine learning engineer with over 12 years of experience specialising in Natural Language Processing (NLP) and deep learning innovation. Dedicated to making your projects succeed.

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4 Comments

  1. Shahrokh

    Hi. I have 5 numbers as the input sequence for example 2,5,10,6, and 8. How can I predict the sixth number using self attention mechanism?

    Reply
    • Neri Van Otten

      Hi,
      Could you provide more information? What kind of data do you have to train on? Do you just have a bunch of numerical sequences?

      Reply
      • Shahrokh

        I have a time series dataset. This dataset includes the number of images enter to the system on a weekly, or on a daily, or in a monthly basis. I want to predict the next number of images in the next time of a day.

        Reply
        • Neri Van Otten

          Hi Shahrokh,
          I would use a transformer architecture as they excel at capturing long-range dependencies within sequences. You can stack several self-attention layers to allow the model to learn different parts of the sequence. Combine this with a feed forward neural network and then implement an appropriate output layer to return the number of images for the next time period. I hope this architecture makes sense.
          Have fun with the implementation 🙂
          Neri

          Reply

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