Add ONNX Runtime GQA-style SDPA benchmark

[kimishpatel]

Stack from ghstack (oldest at bottom):

• Add xplat cxx_binary target for bench_sdpa #18705
• Use unfused SDPA for short sequences (q_len <= 128 or kv_len <= 128) #18651
• Limit softmax to causally-valid elements in cpu_sdpa #18650
• Add quantized input support to cpu_sdpa #18649
• [Executorch] Add non-flash SDPA for decode #18648
• -> Add ONNX Runtime GQA-style SDPA benchmark #18647
• Add GEMM-based standard SDPA benchmark #18646
• Fix SLEEF preprocessor macro name to match ATen vec headers #18645

Add run_onnx_gqa_sdpa() which faithfully ports the algorithm from
onnxruntime/contrib_ops/cpu/bert/gqa_attention_base.h:

• Scale baked into GEMM alpha (no separate scaling pass)
• Scores buffer padded to max_seq_len columns
• Causal mask: zero out future positions, softmax on valid window only
• Output always in [B, S, Hq, D] format

Extends validation to verify ONNX GQA output matches custom_sdpa_out
reference. Adds OnnxGQABenchFixture for benchmarking both layouts.

Differential Revision: D96044317

[pytorch-bot]

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❌ 1 Cancelled Job, 5 Unrelated Failures

As of commit fa4dd36 with merge base 1debeb6 ():

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest / macos / macos-job (gh)

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / unittest / windows / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• Test CoreML Backend / test-coreml / test-backend-macos (coreml, models) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test CoreML Backend / test-coreml / test-backend-macos (coreml, operators) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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