Fix distributed GRPO Recipe

[chivatam]

Context

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• add a new feature
• fix a bug
• update tests and/or documentation
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Please link to any issues this PR addresses.

Changelog

What are the changes made in this PR?

1. Fix trajectory concatenation: Update generate_trajectory_batched to properly handle the 'answers' field which is a list of strings, not a tensor.

2. Update reward function signature: Modify batched_rewards call to include device parameter and handle multiple reward functions properly.

3. Add missing fields to GRPOTrajectory: Add rewards and successes fields to the GRPOTrajectory NamedTuple and update docstring.

4. Fix GRPOStats concatenation: Update the concatenation logic in the training loop to properly handle the metadata field.

5. Fix reward indexing: Correct the answer indexing in batched_rewards function from answers[b][g] to answers[b].

These changes ensure that distributed GRPO training works correctly with proper trajectory handling and reward computation.

Test plan

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UX

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and a tutorial example

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

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[chivatam]

@joecummings
lmk if there are any additional edits!