Add GQA XQA and Flash workspace recipes (#32453)
## Summary
Stacked on #32446, add graph-free checked workspace recipes for the CUDA
GroupQueryAttention decode backends:
- reproduce XQA semaphore and multi-block scratch sizing from plain
device/shape scalars;
- retain XQA RoPE Q/K and dynamic FP32 head-sink conversion allocation
parity;
- reproduce Flash regular and fast-decode LSE/split-accumulator sizing;
- preserve the GQA fast-decode distinction between KV heads used for
split selection and query heads used for accumulator allocation;
- fail closed for unsupported XQA cache/storage configurations.
## Flash bounded-estimation constraint
Flash workspace is not monotonic in KV length. For `B=1`, `S_q=1`, two
heads, head size 64, and 108 SMs, increasing KV length from 13,824 to
13,825 changes the selected split count from 54 to 28 and reduces
workspace. Future Level-1/Level-2 aggregation must compute a
conservative envelope or report unavailable rather than evaluating only
the maximum shape.
The split heuristic matches runtime double-threshold semantics,
including a regression boundary where using `0.85f` would select 17
splits instead of the runtime 20.
## Scope
This PR adds concrete selected-backend recipes only. It does not add
MEA/unfused recipes, complete-route composition, dynamic-bound route
reachability, L1/L2 adapters, cuDNN workspace queries, runtime
allocation changes, or #32071 planner integration.
## Validation
- CUDA provider test target built successfully.
- 201 CUDA internal tests passed; 2 unrelated Lean Attention tests
skipped.
- 44 GQA preparation/XQA/Flash tests passed, including runtime-helper
parity and Flash discontinuity regressions.
Tracking: #29775
Dependency: #32446
> This draft temporarily targets `main` because GitHub cannot use a
fork-only branch as the base of an upstream PR. Its diff will reduce to
this commit after #32446 merges.
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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: b24d04bb-12ab-41a0-8941-76eb9769005d
