Add Sample-level Logging API

[DNXie]

This PR introduces structured sample-level logging to complement existing scalar metrics, allowing users to inspect concrete prompt–response–reward examples during RL training.

More discussion: #301

The current implementation logs 2 samples: one top (highest reward) and one bottom (lowest reward) for each step. Supporting customized sampling strategy is out of scope of this PR. For now, this can be achieved by changing the filter for SampleAccumulator. For example:

class SampleAccumulator(MetricAccumulator):
    """Accumulator for sample-level metrics (e.g., prompt/response/reward dicts).

    Optionally uses a sample filter to decide what to keep at append/flush time.
    """

    def __init__(
        self, reduction: Reduce, filter: TopBottomKFilter | None = TopBottomKFilter(top=2, bottom=2)  # <--- here
    ):

---

Summary of Changes

• Added SampleAccumulator to support logging structured dict samples (e.g., per-episode data) via record_episode_sample API with Reduce.SAMPLE.
• Implemented a simple TopBottomKFilter for selecting top/bottom samples based on reward (heap-based, O(log k) per append).
• Extended both backends:
  • ConsoleBackend: pretty-prints structured samples as JSON.
  • WandbBackend: logs in wandb.Table for live sample inspection.

---

Logged Fields

Each logged sample includes:

{
        "episode_id": episode.episode_id,
        "policy_version": episode.policy_version,
        "prompt": episode.request,
        "response": episode.response,
        "target": str(episode.target),
        **(
            episode.reward_breakdown or {}
        ),  # per-fn breakdown including the average reward
        "advantage": episode.advantage,
        "request_len": episode.request_len,
        "response_len": episode.response_len,
        "pad_id": episode.pad_id,
    }

Tests:

• Unit tests
• Verified with python -m apps.grpo.main --config apps/grpo/qwen3_1_7b.yaml with modes:
  • global_reduce
  • per_rank_reduce: The table is logged in the run Controller_xxx
  • per_rank_no_reduce The table is logged in the run Controller_xxx and with higher latency.

---

Backend: wandb

Backend: Console:

==========  SAMPLE LOGS STEP 1 ==========
[rollout/sample] (2 samples)
{
  "episode_id": "ea2f341b-bf7a-4647-9913-c5f2c8bb525f",
  "policy_version": 0,
  "prompt": "<|im_start|>system\n\n            Put all your scratchpad work between <think> and </think> tags.\n            Your final answer should be between <answer> and </answer> tags otherwise it will not be scored.\n            <|im_end|>\n<|im_start|>user\nJames buys $3000 worth of stuff from Amazon.  He has to return a TV that cost $700 and a bike that cost $500.  He also sells another bike that cost 20% more than the bike he returned for 80% of what he bought it for.  He then buys a toaster for $100.  How much is he out of pocket for everything?<|im_end|>\n<|im_start|>assistant\n",
  "response": "<think>\nOkay, let me try to figure out how much James is out of pocket for everything. Hmm, let's start by breaking down the problem step by step.\n\nFirst, James buys $3000 worth of stuff from Amazon. So, his initial total cost is $3000. But then he has to return a TV that cost $700 and a bike that cost $500. Wait, so when he returns those items, he's getting them back, right? So maybe that means he's losing the value of those items? But I need to check if that's how it's calculated. \n\nBut the question is about how much he's out of pocket. So, when he returns the TV and bike, he's probably losing the amount he paid for them. So, the TV was $700, bike was $500. So, total money he gave back is 700 + 500 = $1200. But he had $3000 initially. So, if he returns $1200, he's out of pocket for those items? Wait, but maybe he has to pay for them back? Or does returning them mean he's just not keeping them? \n\nWait, maybe I need to think in terms of net cash flow. Let me think. When he buys stuff, he spends money. When he returns stuff, he gives money back, so that's like a deduction from his total expenditure. \n\nSo, initial purchase: $3000. Then he returns $700 (TV) and $500 (bike). So, total returned is $1200. Therefore, his net expenditure so far is $3000 - $1200 = $1800. But then he sells another bike that cost 20% more than the bike he returned. Wait, the bike he returned was $500, so the bike he sold is 20% more than that. Let me calculate that. 20% of 500 is 100, so the bike he sold cost 500 + 100 = $600. But he sells it for 80% of what he bought it for. So, he bought it for $600, so 80% of that is 0.8 * 600 = $480. So",
  "target": "2020",
  "MathReward": 0.0,
  "ThinkingReward": 0.2,
  "reward": 0.1,
  "advantage": -7.449986878782511e-05,
  "ref_logprobs": -1.359375,
  "request_len": 512,
  "response_len": 512,
  "pad_id": 151643
}
{
  "episode_id": "b0a1d37d-c3a6-403a-8e9f-8ae510b6702a",
  "policy_version": 0,
  "prompt": "<|im_start|>system\n\n            Put all your scratchpad work between <think> and </think> tags.\n            Your final answer should be between <answer> and </answer> tags otherwise it will not be scored.\n            <|im_end|>\n<|im_start|>user\nJohn uses 5 liters of fuel per km to travel. How many liters of fuel should John plan to use if he plans to travel on two trips of 30 km and 20 km?<|im_end|>\n<|im_start|>assistant\n",
  "response": "<think>\nOkay, let's see. John is traveling twice, first 30 km and then 20 km. He uses 5 liters per km. So I need to calculate the total fuel needed for both trips.\n\nFirst, I should figure out how much fuel he uses for each trip separately and then add them together. For the first trip, 30 km multiplied by 5 liters per km. Let me do that calculation: 30 times 5 is 150 liters. Then for the second trip, 20 km times 5 liters per km. 20 times 5 is 100 liters. Adding those two amounts together, 150 plus 100 equals 250 liters. So John should plan to use 250 liters of fuel in total. I think that's it. Let me double-check. 30 km trip uses 150 liters, 20 km uses 100. Total is indeed 250. Yep, that seems right.\n</think>\n\nJohn uses 5 liters of fuel per km. For the first trip of 30 km, he uses $30 \\times 5 = 150$ liters. For the second trip of 20 km, he uses $20 \\times 5 = 100$ liters. Total fuel required is $150 + 100 = 250$ liters.\n\n<answer>250</answer>",
  "target": "250",
  "MathReward": 1.0,
  "ThinkingReward": 1.0,
  "reward": 1.0,
  "advantage": 0.5398026704788208,
  "ref_logprobs": -4.90625,
  "request_len": 512,
  "response_len": 512,
  "pad_id": 151643
}
==============================================

Notes

• There tend to be a delay on W&B table logging. Typically you'd have to wait for a few more (sometimes over 10) steps for the tables to be visible on wandb.