Limit softmax to causally-valid elements in cpu_sdpa

[kimishpatel]

Stack from ghstack (oldest at bottom):

• temp changes to enable benchmarking on device via adb #18719
• Add xplat cxx_binary target for bench_transposed_cache benchmark #18718
• Add non-flash SDPA path gated by ET_USE_UNFUSED_SDPA #18717
• Add ONNX Runtime GQA-style SDPA benchmark #18716
• Add GEMM-based standard SDPA benchmark and fix custom_sdpa_out signature #18715
• Support separate K/V seq dim in custom_sdpa op #18714
• Add --no_transposed_cache CLI flag for export pipeline #18713
• Plumb transposed cache config through export pipeline #18712
• Add benchmarks for transposed vs standard KV cache layout #18711
• temp hack #18710
• Add support for transposed kv cache in all variants of custom kv cache #18709
• Add contig requirement to aot binding for llm custom ops #18708
• Add support for seq at dim=2 #18707
• Make kv cache update op support transposed cache #18706
• 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

Instead of setting masked positions to -inf and computing
max/exp/normalize over all kvSize elements, limit the softmax
to only the causally-valid range per row. This avoids unnecessary
computation on masked positions and zero-fills them for GEMM 2.

Differential Revision: D96044307

[pytorch-bot]

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❌ 2 New Failures, 2 Cancelled Jobs

As of commit 2e6dc3a with merge base fb1618e ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner (gh)
  >>> Lint for extension/llm/custom_ops/op_sdpa_impl.h:
• pull / unittest-arm-backend-with-no-deps (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t a5282a4da2cda7cc61653df2614bb1822cba67d244329383e615aa4d1b292ef3 /exec failed with exit code 1

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / unittest / windows / windows-job (gh)
• pull / unittest-editable / windows / windows-job (gh)

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