[ASM] [HIP] [CK] feat(mha): gfx950 hd256 FP8 LINEAR paged-varlen asm prefill

[MiloLurati]

Motivation

SGLang chunked prefill uses a paged KV cache (page size 64, LINEAR [N, 64, H, D]). The gfx950 hd256 FP8 asm prefill kernel only accepted contiguous KV, so that path fell back to CK. This PR selects a PAGED_VARLEN asm kernel for that layout.

Technical Details

• op_tests/test_batch_prefill.py: test_batch_prefill_hd256_fp8_page64_asm (LINEAR, causal/non-causal, including one randomized Q/KV length case) and test_batch_prefill_hd256_fp8_page16_stays_ck.
• --pagesize accepts 64.

Test Plan

• op_tests/test_batch_prefill.py: test_batch_prefill_hd256_fp8_page64_asm (gfx950-only skip; LINEAR; causal/non-causal; equal lengths, Q=128/KV=1024, randomized Q/KV with KV ≥ Q, GQA 16:2).
• --pagesize choices include 64 (-p 64 --kv_layout linear for a CLI bench). 64 is not in the default [1, 16, 1024] sweep

Test Result

• page64 asm: 8/8
• CK smoke (LINEAR fp8 hd256 ps=16): 3/3 (SGLang non-causal, SGLang causal+softcap, vLLM)

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

[github-actions]

🏷️ CI Guide

Runs automatically on every PR:

• ✅ Pre-checks (submodule verification, code formatting)
• ✅ Aiter op tests (gfx942 + gfx950)
• ✅ Triton tests on MI35X (only when aiter/ops/triton/** or related paths are changed)

Extended tests (opt-in via labels):

Label	Tests
ci:gfx1250-ffm-triton	Run the five-shard gfx1250 FFM Triton test suite
ci:triton-300x	Run an additional Triton test job on MI300X in PRs; main branch always runs both MI35X and MI300X
ci:sglang	SGLang integration tests: DeepSeek-R1-MXFP4 accuracy, Qwen 3.5 accuracy
ci:atom	ATOM benchmark: DeepSeek-R1-0528, GPT-OSS-120B
ci:atom_full	ATOM accuracy suite for PR and main models from ATOM models_accuracy.json
ci:vllm	vLLM benchmark: GPT-OSS-120B, DeepSeek-R1-0528, Kimi-K2.5
ci:all	All standard extended tests (excludes ci:atom_full)

Only add ci:atom_full for FlyDSL or Triton upgrades.
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