Update on "Add ONNX Runtime GQA-style SDPA benchmark"

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](https://our.internmc.facebook.com/intern/diff/D96044317/)

[ghstack-poisoned]

Author: kimishpatel