Update base for Update on "Add GEMM-based standard SDPA benchmark"

Add bench_sdpa.cpp with a standalone GEMM-based SDPA implementation
(run_standard_sdpa) alongside ExecuTorch's tiled flash attention
(custom_sdpa_out) for comparative benchmarking.

The standalone SDPA uses full GEMM per head with 3-pass softmax and
supports both [B,S,H,D] and [B,H,S,D] layouts via BLAS leading
dimension parameters, allowing isolation of algorithm vs layout effects.

Includes validation tests that verify the GEMM-based implementation
matches custom_sdpa_out within tolerance.

Differential Revision: [D96044313](https://our.internmc.facebook.com/intern/diff/D96044313/)

[ghstack-poisoned]

Author: kimishpatel