Add FP16 and BF16 H512 GQA decode support (#33176)
### Description
- Add FP16/BF16 H512 XQA decode kernels with FP32 online softmax and
partial-result merging.
- Support BNSH/BSNH caches, integral positive GQA ratios, local windows,
and attention sinks.
- Align runtime scratch geometry, workspace recipes, bounds, estimation,
and eligibility checks.
- Prefer native grouped H512 attention where eligible instead of
oversized expanded-KV fallback workspaces.
- Add documented kernel, eligibility, workspace, and cache-immutability
regressions; retain existing smaller-head dispatch paths.
- Preserve main's shared route-eligibility helpers and integrate H512
geometry into the shared XQA predicate.
### Motivation and Context
Gemma 4 grouped attention includes 512-wide heads. This adds a bounded
decode implementation and consistent workspace accounting rather than
expanding the cache to query-head count.
### Validation
After merging main at `05517e129656033b13a1617f1d94d0bdc9f0dff1`,
resolved commit `e4dedff734cadcc2df828f176aaecf2a54ce3312` was rebuilt
in an isolated Release CPU/CUDA build on Linux/A100 (SM80), CUDA 13,
QUICK_BUILD off. All 90 native GQA operator tests and 106 focused
internal CUDA kernel/workspace/eligibility tests passed. The 13 operator
skips are 12 unavailable WebGPU tests and one FP8 test requiring SM89+.
Both direct FP16/BF16 H512 kernel tests and all four typed instances of
the new unquantized-H512 geometry regression ran and passed. All 17 PR
files pass configured lint and whitespace checks. No full-suite,
Windows, ROCm, or other GPU-architecture validation.
This is one of four independent ORT drafts. Prefer merging the companion
GQA cache/window draft
https://github.com/microsoft/onnxruntime/pull/33175 first so the native
fallback also receives its window-bound correction; this branch was
built and tested without that draft applied.
### Conflict Resolution
Merged main without rewriting the original tested commit. The resolution
retains main's XQA and MEA helper refactors, adds H512 geometry to the
shared XQA predicate, keeps the native-H512 MEA exclusion, and preserves
both operator tests that collided at the same insertion point. No other
companion draft commits were applied.
### Latest Main Merge (2026-10-09)
Resolved the current conflict by merging main at
`e379df24dbe4b0c6912282f30fe3f5ae4c1836d4` in
`d73ed2d1538ed3dc02a2762057156c122eb6eb00` without rewriting branch
history. The operator-test conflict was two independent insertions at
the same location: both the native H512 fallback eligibility regression
and the upstream FlashAttention L2-cache block-size regressions are
preserved, with their original platform guards. Upstream GQA runtime
changes merged automatically.
For this merge, rebuilt the resolved GQA test translation unit against
the merged H512 headers and ran all three conflict-local header-only
helper tests successfully. The test executable reused retained backend
libraries; this is not a fresh H512 numerical-kernel validation. The
automatically merged CUDA GQA runtime source passed a compile check. All
17 remaining PR files passed configured lint and whitespace checks, and
editor diagnostics reported no errors in the resolved file.
The full native H512 runtime results above describe the earlier
validated revision; those numerical tests and a full native rebuild were
not rerun for this merge. CI for the new merge commit remains pending.
