perf(rocm): reduce Kimi-K3 attention into stable outputs

[JohnQinAMD]

Summary

Add a fail-closed all_reduce_into path for caller-owned output, thread the
optional destination through RowParallelLinear, and let AMD Kimi-K3 KDA and
MLA reduce directly into graph-stable decoder buffers.

This PR is stacked on #14 and depends on the companion AITER
caller-output support in JohnQinAMD/aiter-amd#27.

Design and fallback

• Existing callers keep output=None; their allocation and collective path is
  unchanged.
• Only AMD Kimi-K3 KDA and MLA pass a destination.
• The common KDA class exposes a protected output hook whose default preserves
  the existing projection-plus-copy; the AMD subclass overrides only that hook.
• Older AITER builds and unsupported collectives use the existing all-reduce
  followed by a correctness-preserving copy.
• Shape, dtype, device, and contiguous-layout mismatches fail before dispatch.

NVIDIA Kimi-K3 call sites are unchanged.

Performance

The decoder previously launched 93 attention-output copyBuffer operations per
token across 69 KDA and 24 MLA layers. The candidate trace has zero.

Kimi-K3 TP8, batch 1, 8K input / 1K output, no speculative decoding on 8x
MI355X:

Stack	Decode	Mean TPOT
Parent	69.7791 tok/s	14.3310 ms
Candidate mean	71.8086 tok/s	13.9259 ms

Candidate runs were 71.8136 and 71.8037 tok/s: +2.91% and -0.4051 ms/token.

Accuracy and validation

• Frozen GSM8K first 100: candidate 100/100, zero invalid; parent prefix
  99/100.
• Focused vLLM tests: 10 passed; combined AITER/vLLM tests: 16 passed.
• Full and piecewise TP8 graph capture passed.
• Eight-rank trace confirmed 93 -> 0 output-copy launches.
• Current-main changed-file pre-commit passed, including Ruff, format, Python
  3.10 mypy, SPDX, import guards, configuration checks, and CUDA API guard.
• git diff --check passed.

Upstream overlap

vllm-project#50592 returns the KDA projection allocation directly. It does
not add caller-owned all-reduce output, graph-stable destinations, MLA support,
or the older-AITER fallback. The changes pursue the same KDA copy but are not
additive; this PR documents the broader measured ownership contract and must be
reconciled if vllm-project#50592 lands first.

Disclosure

Developed with assistance from OpenAI Codex. The author reviewed the API
boundary, fallbacks, tests, graph trace, accuracy, and endpoint measurements.

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