Add GEMM-based standard SDPA benchmark and fix custom_sdpa_out signature

Adds a standalone GEMM-based (non-tiled) attention benchmark alongside
the existing ET flash attention benchmark, allowing direct comparison
of the two algorithms. The standard SDPA uses cblas_sgemm for Q@K^T
and scores@V with 3-pass softmax, matching the approach used by
ONNX Runtime's GQA operator.

Both transposed [B,H,S,D] and standard [B,S,H,D] cache layouts are
supported via BLAS leading dimension parameter. Validation tests run
before benchmarks to ensure correctness against ET's custom_sdpa_out.

Also fixes broken custom_sdpa_out calls to match the new 3-bool
signature (is_seq_dim_2, is_k_seq_dim_2, is_v_seq_dim_2).

Authored with Claude.

Differential Revision: [D99677686](https://our.internmc.facebook.com/intern/diff/D99677686/)

[ghstack-poisoned]

Author: kimishpatel

extension/llm/custom_ops/BUCK

@@ -139,6 +139,8 @@ cpp_benchmark(
     deps = [
         "fbsource//third-party/benchmark:benchmark",
         "//executorch/extension/llm/custom_ops:custom_ops_mkl_noomp",
+        "//executorch/extension/threadpool:threadpool",
+        "//executorch/kernels/optimized:libblas",
         "//executorch/runtime/core/exec_aten:lib",
         "//executorch/runtime/core/exec_aten/testing_util:tensor_util",
     ],