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A Simple Overview of the LLM Training Steps ๐Ÿ”ก

ยท 2 min read
Zain Hasan

A preview of the paper

A Simple Overview of the LLM Training Steps:๐Ÿ”ก

  1. Unsupervised Pretraining:

    High quantity, low quality data The model is trained to predict the next token for trillions of tokens. Produces what is called the foundation or base model.

  2. Supervised Finetuning:

    Low quantity, high quality {prompt, response} Enables the model to be finetuned for dialogue - turning the base model into a chatbot Often referred to as instruction tuning

  3. Reinforcement Learning from Human Feedback (RLHF): - lots of innovation going on here (will cover DPO, PTO, and KTO soon)

This is a two-step process:

a. Train a reward model to act as a scoring function:

This model will take in a prompt + response and provide a score of how good it is. Human labelers are asked to pick good vs. bad responses and this data is used to train a model.

b. Optimize LLM to generate responses for which the reward model will give high scores.

Use an iterative procedure to update a part of the model such that:

  1. Produces outputs with higher score
  2. Outputs that are not too far away from the SFT model from Step 2
  3. Outputs that aren't getting worse a text completion

Specifically for this phase it is better to think of this as learning an optimal strategy/policy for predicting a probability distribution over tokens and we want to tweak this distribution to produce higher quality text, here the:

The policy is a language model that takes in a prompt and returns a probability distribution over text. The action space of this policy is all the tokens corresponding to the vocabulary of the language model (~50k tokens) The observation space: distribution of possible input token sequences The reward Model is a combination of the preference model(score higher) and a constraint on policy shift(don't change too much, get worse at text completion).

RLHF Learning Resources:

  1. InstructGPT Paper

  2. RLHF Paper Anthropic

  3. OpenAI Blog

  4. RLHF Blog Chip Huyen

  5. RLHF Nathan Lambert

  6. Karpathy Talk

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