Add benchmarks for transposed vs standard KV cache layout

[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

Benchmarks comparing transposed [B, H, S, D] vs standard [B, S, H, D] KV
cache layouts in custom_sdpa and update_cache ops using Llama 3 8B config
(32 Q heads, 8 KV heads, D=128). Both C++ (Google Benchmark) and Python
benchmarks are included, covering decode (seq_len=1) at various cache fill
levels and prefill scenarios.

Results on Apple M-series show transposed cache significantly improves SDPA
performance at longer cache fills (1.64x at start_pos=1024, 1.13x for
prefill seq_len=512) due to better memory locality in the attn_score @ V
GEMM — V stride along S_kv changes from H*D to D.

Authored with Claude.

Differential Revision: D99677680

[pytorch-bot]

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❌ 9 New Failures, 2 Cancelled Jobs

As of commit 2bd6e06 with merge base fb1618e ():

NEW FAILURES - The following jobs have failed:

• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 1d9b8538270256f251d792d6e2df2c4d1c5e49cf1a262b7569913afb92f09b83 /exec failed with exit code 1
• pull / test-lora-multimethod-linux / linux-job (gh)
  RuntimeError: Command docker exec -t f152220a0d4e5c3306e658987eec3d052d86a6ac523facbb3a23eb0fad914cdf /exec failed with exit code 1
• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 8b3b2e848e8025d24594cca90ad4e597ae94177b0e34a56093dd6ad2bd5a57e0 /exec failed with exit code 139
• pull / unittest / linux / linux-job (gh)
  examples/models/llama/tests/test_replace_kv_cache.py::TestReplaceKVCache::test_replace_quantized_kv_cache_with_quantized_ring_kv_cache
• pull / unittest / macos / macos-job (gh)
  examples/models/llama/tests/test_replace_kv_cache.py::TestReplaceKVCache::test_replace_quantized_kv_cache_with_quantized_ring_kv_cache
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 9c648d61c7e0743dd6f9271535708766e9f4e9fe4cdeaa5611573f7136859713 /exec failed with exit code 3
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3
• pull / unittest-editable / linux / linux-job (gh)
  examples/models/llama/tests/test_replace_kv_cache.py::TestReplaceKVCache::test_replace_quantized_kv_cache_with_quantized_ring_kv_cache
• pull / unittest-editable / macos / macos-job (gh)
  examples/models/llama/tests/test_replace_kv_cache.py::TestReplaceKVCache::test_replace_quantized_kv_cache_with_quantized_ring_kv_cache

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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[kimishpatel]

submitted by accident, not meant to land immedidately