Building GPT-2 from Scratch: A Complete LLM Teaching Project

This article introduces the open-source project "LLM-from-scratch", which implements the GPT-2 architecture from scratch using PyTorch. It includes core Transformer components, a dual-pipeline fine-tuning system (spam classifier and conversational assistant), as well as supporting web interfaces and deployment solutions, helping learners deeply understand the underlying principles of LLMs.

Most current LLM tutorials stay at the level of API calls or ready-made frameworks, making it difficult for learners to understand internal mechanisms. This project adopts the "from scratch" methodology, requiring hands-on implementation of core components such as word embeddings and multi-head attention. The author believes that only by personally implementing positional encoding and experiencing gradient propagation can one truly understand the design logic of GPT-2.

The project's core file ch04.py fully implements GPT-2 without relying on advanced libraries:

1. Word Embedding and Positional Encoding: Word embeddings are mapped to 768-dimensional vectors, and positional encoding adds positional information;
2. Multi-Head Self-Attention: Implements Query/Key/Value transformation, scaled dot-product attention, and masking mechanism;
3. Layer Normalization and Feed-Forward Network: Transformer blocks include residual connections + layer normalization, and the feed-forward network uses GeLU activation;
4. Weight Loading: Provides the gpt_download.py tool to load OpenAI pre-trained weights, supporting self-training or fine-tuning.

The project provides two fine-tuning paths:

• Pipeline A (SpamShield Spam Classification): Freezes most parameters, replaces the output head with a binary classification head, and achieves an accuracy of over 98% when fine-tuned on the UCI dataset;
• Pipeline B (Assistant GPT Conversational Assistant): Modifies GPT-2 Medium with supervised fine-tuning, masks the loss of instruction tokens via a custom collate_fn, and focuses on response generation.

The project equips the two applications with web interfaces:

• SpamShield: Glassmorphism style, real-time spam detection;
• Assistant GPT: ChatGPT-like conversational interface, supporting streaming responses; Deployment solutions include three types: local Ngrok tunnel, Hugging Face Spaces hosting, cloud servers (AWS/DigitalOcean), and provides Git LFS to solve model size issues.

The project has a clear file structure:

• ch02.py: Vocabulary construction and tokenization;
• ch04.py: GPT-2 architecture;
• spamClass.py/pers.py: Classification/instruction fine-tuning scripts;
• app.py/assistant_app.py: Web backend; Recommended learning sequence: First understand the ch04.py architecture, then experience classification fine-tuning, finally try instruction fine-tuning, and observe the effects with the web interface.

The project's value lies in the completeness of its teaching design, answering the question "What does it take to train a ChatGPT-like model from scratch?" By implementing components hands-on, developers can build intuition about:

• Why Transformers are more suitable for long texts than RNNs;
• The necessity of the pre-training + fine-tuning paradigm;
• The principles behind conversational models following instructions; These understandings help engineers design prompts, choose fine-tuning strategies, and diagnose bad cases.

"LLM-from-scratch" is a high-quality teaching project suitable for researchers to deeply understand Transformers or engineers to master fine-tuning techniques. In the LLM era, the gap between developers who "understand the principles" and those who "only know how to call APIs" will widen, and this project provides an excellent starting point for building technical competitiveness.