RLHF (Reinforcement Learning From Human Feedback): Overview + Tutorial

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What is Reinforcement Learning with Human Feedback (RLHF)?

The concept of RLHF is simple: use a pretrained language model, then have humans assess and score the responses it generates. This human-curated ranking acts as a powerful signal to the model, directing it to favor certain outputs over others, encouraging the production of answers that are more dependable, more secure, and more in line with human expectations.

RLHF improves the model's understanding and generation of natural language by incorporating human feedback. Furthermore, it boosts its ability to execute particular jobs with greater precision, such as text classification or language translation.

It is possible to direct the model towards more fair and inclusive language use by incorporating human evaluators into the training loop, addressing biases in the machine's outputs. It is essential to remember, however, that human involvement can be a double-edged sword, as it might introduce biases from the human evaluators themselves.

Let's take a closer look at this groundbreaking method, how it works, and its significance in the ever-changing realm of artificial intelligence.

The roots of Reinforcement Learning (RL)

Before we fully understand RLHF, we must first grasp the fundamentals of Reinforcement Learning (RL).

Reinforcement learning is a subset of machine learning where an agent learns to make decisions by interacting with its environment. It performs actions, affecting its environment, which subsequently transitions to a new state and returns a reward. This reward acts as feedback that helps the RL agent adjust its decision-making policy. Over multiple training episodes, the agent refines its policy to maximize its total reward.

Read more: The Beginner's Guide to Deep Reinforcement Learning

However, designing an effective reward system poses a significant challenge in reinforcement learning. In many applications, the reward is delayed, or in some cases, defining it with a mathematical or logical formula is not feasible.

Introducing humans into the equation

This is where RLHF comes into play. It augments the RL agent's training by including humans in the process, accounting for elements that can't be quantified in the automated reward system. However, scaling RLHF can be challenging as the involvement of humans in the training process can become a bottleneck. To overcome this, most RLHF systems yse a mix of automated and human-provided reward signals.

In an RLHF system, a human supervisor either provides an occasional extra reward/punishment signal or offers the data needed to train a reward model. This hybrid approach offers a balance between the scalability of machine learning and the nuanced understanding of human evaluators.

RLHF and Large Language Models (LLMs)

One area where RLHF has shown significant promise are Large Language Models (LLMs). LLMs, such as GPT-3, are excellent at various tasks, including text summarization, question answering, and more. Yet, they share a fundamental limitation—the inability to align the model with all possible applications it will be used in.

RLHF can help steer LLMs in the right direction by defining language as a reinforcement learning problem:

1. Agent: The language model acts as the reinforcement learning agent and learns to produce optimal text output.

2. Action Space: The set of possible language outputs that the LLM can generate.

3. State Space: Includes the user prompts and the outputs of the LLM.

4. Reward: Measures the alignment of the LLM's response with the application's context and the user's intent.

However, the reward system in this setup is not well defined. This is where Reinforcement Learning From Human Feedback proves invaluable, allowing the creation of a robust reward system for our language model.

Read more: Large Language Models (LLMs): Challenges, Predictions, Tutorial

RLHF training process: a step-by-step breakdown

As we delve deeper into the intricate mechanisms of the Reinforcement Learning from Human Feedback (RLHF) algorithm, it's important to keep in mind the relation between the algorithm and its foundational component: the pretraining of a Language Model (LM).

Let’s break down each step of the RLHF algorithm.

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Pretraining a language model (LM)

The pretraining process forms the foundation for RLHF. At this stage, a language model (LM) is trained on a large corpus of text data collected from the internet. This data helps the LM understand various nuances of human language, including syntax, semantics, and even some context-specific information.

Step 1: Selecting a base language model

The initial selection of a base language model is crucial for the RLHF process. There isn't a universally best model to start with—the choice largely depends on the specific task at hand, available resources, and the unique challenges of the problem space. Industry practices vary greatly, with organizations like OpenAI using a smaller version of GPT-3 for their RLHF model, InstructGPT, and Anthropic and DeepMind opting for models ranging from 10 million to 280 billion parameters.

Step 2: Gathering and preprocessing data

For RLHF, the selected LM is pretrained on an extensive dataset, which typically consists of large quantities of text from the internet. This raw data must be cleaned and preprocessed to be suitable for training, a process that often involves removing unwanted characters, correcting errors, and normalizing textural irregularities.

Read more: A Simple Guide to Data Preprocessing in Machine Learning

Step 3: Training the language model

The LM is then trained on this dataset, learning to predict the next word in a sentence given the preceding words. This stage involves the optimization of model parameters using methods such as stochastic gradient descent. The ultimate goal is to minimize the difference between the model's predictions and the actual data, a measure often calculated using a loss function, such as cross-entropy.

Step 4: Evaluating the model

Post-training, the model is evaluated on a held-out dataset, separate from the one used for training. This step is necessary to ensure that the model can generalize well and hasn't merely memorized the training data. If the evaluation metrics are satisfactory, the model is deemed ready for the next phase of RLHF.

Read more: Top Performance Metrics in Machine Learning: A Comprehensive Guide

Step 5: Preparing for RLHF

While the LM has learned a lot about human language at this point, it still lacks knowledge of human preferences. To incorporate this, additional data is needed. Often, companies pay humans to generate responses to prompts, which are then used to train a reward model. This step can be expensive and time-consuming, but it's vital for steering the model in the direction of human-like preferences.

Note that the pretraining phase doesn't result in a perfect model; the model is expected to make mistakes and generate incorrect outputs. However, it provides a substantial starting point upon which RLHF can build, making the model more accurate, safe, and useful.

Training a reward model

The cornerstone of the RLHF process is the creation and training of a reward model (RM). This model essentially acts as an alignment tool, offering a way to integrate human preferences into the AI's learning process.

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Step 1: Establishing the reward model

The reward model can either be an end-to-end language model or a modular system. Its primary function is to map input text sequences to a scalar reward value, a system that enables reinforcement learning algorithms to optimize over time within their environment.

For instance, if an AI generates two different text responses, the reward model will determine which one aligns more closely with human preferences, essentially 'rewarding' the more appropriate output.

Step 2: Data collection

Training the reward model begins with a specific dataset, distinct from the one used in language model pretraining. This dataset is composed of prompt and reward pairs and is narrower in scope, focusing on the specific use cases.

Each prompt corresponds to an expected output, with associated rewards to indicate the desirability of the output. While this dataset is typically much smaller than the initial pretraining dataset, it is critical in guiding the model to produce user-aligned content.

Step 3: Model training

Using the prompt and reward pairs, the model is trained to associate specific outputs with their respective reward values. This process often leverages large 'teacher' models or ensembles thereof to enhance diversity and mitigate bias. The aim here is to develop a reward model capable of accurately determining the desirability of potential output.

Step 4: Human feedback integration

A key element of training the reward model is the inclusion of human feedback. A perfect example of this is ChatGPT, where users can rank the AI's output using a thumbs-up or thumbs-down system.

This crowdsourced feedback is invaluable in refining the reward model, as it gives direct insight into human preferences. It is through this iterative cycle of model training and human feedback that AI continues to evolve, enhancing its alignment with human preferences.

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Fine-tuning the language model with reinforcement learning

Fine-tuning is an essential part of the Reinforcement Learning with Human Feedback process. It facilitates the training of a language model so that it can generate more appropriate responses based on user prompts. This process is accomplished through the use of reinforcement learning techniques, including the utilization of Kullback-Leibler (KL) divergence and Proximal Policy Optimization (PPO).

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Step 1: Utilizing the reward model

Initially, a user input, or prompt, is sent to the RL policy, essentially a tuned version of the LM. The RL policy generates a response, which, along with the initial LM's output, is assessed by the reward model. This model then generates a scalar reward value corresponding to the quality of the responses.

Step 2: Introducing the feedback loop

This process is iterated in a feedback loop where the reward model assigns rewards to as many samples as resources permit. Over time, the responses that receive higher rewards will guide the RL policy, helping it to generate more human-aligned responses.

Step 3: Measuring differences with KL Divergence

Kullback-Leibler (KL) Divergence, a statistical method used to measure the difference between two probability distributions, plays a vital role here. In RLHF, KL Divergence is utilized to compare the probability distribution of the RL policy's current responses with a reference distribution that represents the ideal, or most human-aligned, responses.

Step 4: Fine-tuning with Proximal Policy Optimization

A significant part of the fine-tuning process involves Proximal Policy Optimization (PPO). PPO is a renowned reinforcement learning algorithm known for its effectiveness in optimizing policies in complex environments with high-dimensional state and action spaces. PPO is particularly useful in the RLHF fine-tuning process as it effectively balances exploration and exploitation during training. This balance is essential for RLHF agents, as they need to learn from both human feedback and trial-and-error exploration. As a result, the integration of PPO can lead to faster and more robust learning.

Step 5: Discouraging inappropriate responses

The fine-tuning process helps deters the language model from producing inappropriate or nonsensical outputs. As responses with low rewards are less likely to be repeated, the language model is driven to produce outputs that align more closely with human expectations.