Reimplement `batched_rewards` to fix compatibility, broken in #2698

[krammnic]

Context

What is the purpose of this PR? Is it to

• [ ] add a new feature
• [X] fix a bug
• [ ] update tests and/or documentation
• [ ] other (please add here)

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Changelog

What are the changes made in this PR?

• #2698 #2858

With introduction of async GRPO, the structure of reward modeling was changed leading to some breaks in common dev/grpo_full_finetune_distributed which is used and not deprecated. The idea is to not just revert some old code, but to reimplement it to make the working recipe.

Test plan

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UX

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• [ ] I did not change any public API
• [ ] I have added an example to docs or docstrings

[pytorch-bot[bot]]

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:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/2876

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[hammoudhasan]

@krammnic if this is ready for testing I could give it a shot :) Let me know.

[krammnic]

Still need few updates and it will be ready