Lecture 7: Transformer Models: The Backbone of Modern NLP

Good afternoon, class. Today, we're diving into the heart of modern Natural Language Processing (NLP) - Transformer models. These models, including variants like GPT-4, are pivotal in building AI language models and have been instrumental in AI's recent strides.
What are Transformer Models?
At the core, Transformer models are a type of architecture used in machine learning for handling sequential data. Introduced in 2017 in a paper titled "Attention is All You Need" by Vaswani et al., Transformer models revolutionized NLP by addressing key limitations in previous models like RNNs and LSTMs, namely long training times and difficulty handling long-range dependencies in data.
Where do Transformers Fit in the Technology Stack?
In the technology stack for building an AI language model, Transformers sit in the middle, encapsulating the model's core logic. They take processed and vectorized input data (usually coming from an embedding layer) and transform it into a higher-level representation. This output can then be passed to a task-specific layer, like a classifier or sequence generator.
In essence, Transformers perform the "thinking" part of the AI language model - understanding and interpreting the input data.
How do Transformers Work?
Transformer models' key innovation is the "attention mechanism" which lets them weigh the importance of different words when processing a sentence. Each word is analyzed in the context of all other words, instead of in a fixed window or order, allowing for a more nuanced understanding of text.
The two primary components of Transformers are the Encoder and Decoder. However, some Transformer variants, like BERT, only use the Encoder, while GPT only uses the Decoder.
Encoder: The Encoder takes in the input data and creates a rich, context-aware representation for each word or token.
Decoder: The Decoder takes these representations and generates the output, one word/token at a time, using information from previously generated words and the Encoder's output.
Transformer in Action: Python Code
Now, let's see a simple implementation of a Transformer model using the Hugging Face's transformers library. This code will demonstrate how to use a pre-trained Transformer for a text classification task.
First, we need to install the library:
pythonCopy code
!pip install transformers

Then, we can load a pre-trained transformer and use it to encode some text:
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from transformers import AutoTokenizer, AutoModelForSequenceClassification
import torch

# Load the pre-trained model
tokenizer = AutoTokenizer.from_pretrained("bert-base-uncased")
model = AutoModelForSequenceClassification.from_pretrained("bert-base-uncased", num_labels=2)

# Some example text
text = "Hello, world! This is a transformer model."

# Encode the text into input IDs and attention masks
inputs = tokenizer(text, padding="max_length", truncation=True, max_length=128, return_tensors='pt')

# Run the model
outputs = model(**inputs)

# Get the model's predictions
predictions = torch.nn.functional.softmax(outputs.logits, dim=-1)

This code uses a pre-trained BERT model from Hugging Face's model hub, encodes some text into a format BERT can understand, runs the model, and then outputs the model's predictions.
In practice, you'd want to train this model on your task-specific data, which would require a dataset, a training loop, and some way of evaluating the model's performance. But we'll get into those topics in a future lecture.
Today, we've scratched the surface of Transformer models. However, there's a lot more depth to these models, including self-attention, positional encoding, and various training techniques. We'll delve into these topics in upcoming sessions. For now, keep exploring, and don't hesitate to reach out with any questions!
Lecture 8: The Magic Behind Transformers - Attention Heads & Self-Attention Mechanism
Hello again, class. Today, we're going deeper into the workings of Transformer models by discussing the self-attention mechanism and attention heads - the heart and soul of the Transformer architecture.
Understanding Attention Mechanisms
Attention mechanisms in NLP model the idea of focusing on certain parts of the input data that are more relevant to the task at hand. Think of when you read a long article: you pay more 'attention' to the sentences that carry the main idea, and less to minor details. Attention mechanisms equip our models with a similar ability.
What is Self-Attention?
Self-Attention, also known as intra-attention, is an attention mechanism that relates different positions of a single sequence to compute a representation of the sequence. In simpler words, it's about understanding a word in the context of all other words in the sentence.
Attention Heads
An attention head is a particular component of the self-attention mechanism. Each head will learn different types of attention depending on the training data and task, making the Transformer model highly flexible. For example, one head might focus on syntactic relationships, another on semantic relationships, and another on certain key phrases or words.
How Does Self-Attention Work in Transformers?
In Transformers, the self-attention mechanism computes three vectors for each word - Query (Q), Key (K), and Value (V).
Q, K, V Generation: These vectors are generated by applying different learned linear transformations to the word's input representation.
Score Calculation: Then, the model calculates a score for each word by taking the dot product of its Query vector with the Key vector of every other word.
Normalization & Weight Calculation: The scores are scaled down (to avoid large value issues), and a softmax is applied to get the weights, which represent how much 'attention' each word should get.
Output Vector Calculation: Finally, the output vector for each word is calculated by weighting the Value vectors of all words by the calculated weights and summing them up.
Self-Attention in Action: Python Code
Let's understand the self-attention mechanism with some Python code. For simplicity, we'll implement a scaled dot-product attention function, which is at the core of the Transformer's self-attention mechanism.

import numpy as np
import torch
import torch.nn.functional as F

def scaled_dot_product_attention(query, key, value):
# Calculate Q.K (scaled by sqrt of d_k)
scores = query.matmul(key.transpose(-2, -1)) / np.sqrt(query.size(-1))
# Apply softmax to get weights
weights = F.softmax(scores, dim=-1)
# Calculate the weighted sum of V
output = weights.matmul(value)
return output, weights

# Test our function
# Initialize Q, K, V as random tensors
query = torch.rand(10, 50)
key = torch.rand(10, 50)
value = torch.rand(10, 50)

output, weights = scaled_dot_product_attention(query, key, value)


In this code, we're implementing a basic form of self-attention as a function.
We then create some example Q, K, and V tensors, pass them to our function, and print the resulting output and weights.
Next time, we'll continue our journey deeper into the Transformer architecture, discussing topics like multi-head attention, positional encoding, and how to train a Transformer model for a specific task. Keep up the good work and see you all then!
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