Add GQA workspace estimation (#32617)
## Description
Adds conservative Level-1 and Level-2 workspace estimation for CUDA
contrib `GroupQueryAttention`, tracking #29775.
This is PR4, the final GQA workspace-estimation PR in the current stack.
It depends on #32602, which bounds windowed runtime workspace by the
resident/staged KV extent.
The estimator:
- inventories all reachable GQA CUDA backend routes over dynamic shape
bounds;
- composes checked preparation and backend workspace recipes;
- uses `C` for single-token windowed decode and checked `C + S` for
multi-token staging;
- bounds Flash split workspace independently of the non-monotonic split
heuristic;
- reports non-windowed and reachable cuDNN cases as unavailable when no
sound graph-free bound exists;
- reports Level-1 `runtime_workspace_bytes`; and
- declares one operator-owned slot-0 Level-2 root with 256-byte
alignment.
This PR does not change runtime allocation topology, opt into
planner-owned allocation, or slice a planned root. Those remain
follow-up integration work.
## Validation
- Fresh Release CUDA build on A100 (SM80)
- CUDA internal module: 264 passed, 2 unrelated LeanAttention skips
- GQA workspace estimate/bounds tests: 18/18 passed
- GQA workspace recipe/arithmetic tests: 19/19 passed
- Lintrunner and `git diff --check` passed
- Deep route-dominance review swept 2.7M complete routes and 25.6M Flash
boundary points with no underestimation
## Stack
- Depends on #32602
- Tracks #29775
---------
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
