[Examples] boiler plate code for multi-turn reward for RLHF

[rghosh08]

Description

This PR addresses: [Feature Request] multi-turn reward for RLHF #2271

This PR implements the reward system for multi-turn reinforcement learning from human feedback (RLHF), following the guidelines outlined in the paper Multi-turn Reinforcement Learning from Preference Human Feedback. The key changes involve creating a simulated multi-turn dialogue environment where human feedback (rewards) is used to guide policy learning. The implemented policy is trained using policy gradient methods, updating based on human feedback provided at each turn.

Changes include:

• A multi-turn dialogue environment that simulates five turns of conversation, with human feedback as rewards.
• A policy network to generate responses in the dialogue based on the current conversation state, where input states are padded/truncated to ensure proper dimensionality.
• A reward mechanism that simulates human feedback using random choices for rewards (+1 for positive, -1 for negative feedback).
• A training loop using policy gradients to train the policy network based on discounted cumulative rewards.

Motivation and Context

This change is necessary to replicate the reward structure proposed in the referenced paper, implementing multi-turn RLHF in a way that closely follows the described methodology. It introduces the simulation of human preferences, which plays a key role in the learning process. This change also resolves issue #2271, which proposed adding this reward mechanism to the project.

• Closes [Feature Request] multi-turn reward for RLHF #2271

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of examples)

Checklist

Go over all the following points, and put an x in all the boxes that apply.

• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.

[pytorch-bot]

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[rghosh08]

@vmoens I was wondering when this PR could be merged. Please let me know if there is any gap. Thanks

[vmoens]

Hi @rghosh08
Thanks for working on this.
I appreciate the effort in doing this and it'd be awesome to have some version of this in the lib.

The current script doesn't integrate any component of the library and therefore is of limited value within torchrl.
The RLHF examples we provide are there to show how the libs components are to be used to achieve some specific goal.
I feel the code as it is now would be an odd one out with limited benefit for the user base.
If you're interested I'd invite you to work on editing this for a better integration within the lib!

Thanks again for collaborating

[rghosh08]

Thanks @vmoens for your feedback. I will come up with an integration. Appreciate your guidance.