add retrival reward idea proof of concept

[AlexWortega]

Here is a working proof of concept of my idea about retrival reward, its using sentence transformer to compare generated answer and ground true answer. WB metrics https://wandb.ai/alexwortega/trl/runs/cszh2ve8?workspace=user-alexwortega

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[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[kashif]

@AlexWortega in your sft function I believe you are missing an optimizer.zero_grad()

[github-actions[bot]]

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

[lvwerra]

Closing for now since there is not a lot of activity right now. Feel free to reopen :)