[Bugfix] Wire iteration_stats through StagePool.process_llm_raw_outputs

[amanchugh89]

What

StagePool.process_llm_raw_outputs accepts an iteration_stats arg but overwrites it with a fresh
IterationStats() before calling the output processor, so the processor fills a throwaway and the
caller's object stays empty.

The orchestrator passes its own IterationStats and then records it via OmniPrometheusStatLogger
(inherits PrometheusStatLogger.record, which reads it). So per-(stage, replica) prompt/generation
token counts, preemption / corrupted-request counters, and TTFT/TPOT/ITL histograms are recorded as
zero whenever Prometheus logging is enabled.

Fix

iteration_stats = iteration_stats or IterationStats()

Allocate only when the caller passes None; otherwise the processor fills the caller's object.

Tests

tests/engine/test_stage_pool_iteration_stats.py — asserts the caller's IterationStats reaches
process_outputs by identity (fails on main, passes with the fix), plus the None path. ruff
clean; suite runs in CI.

Not a duplicate: no open PR touches this wiring; the arg was added in #3628 but never threaded
through. AI assistance was used; every line reviewed.

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[amanchugh89]

Validated the regression on the actual code (vLLM 0.23.0): with the fix, tests/engine/test_stage_pool_iteration_stats.py passes (2 passed); reverting just the one-line change makes test_caller_iteration_stats_not_clobbered fail (1 failed, 1 passed) — the identity assertion catches the clobber while the None-path test is unaffected. So it's a genuine red→green regression, not only the reasoning above.

[amanchugh89]

@tzhouam @yenuo26 if you guys have have a pass at this PR ?

[amanchugh89]

Closing for now — hasn't attracted review and I'm focusing elsewhere. The fix + regression test are validated (red→green) if anyone wants to pick it up; happy to reopen. Thanks!