Ship the XNNPACK delegate as its own library

[shoumikhin]

The XNNPACK delegate is what makes many models run fast on CPU. Today
it is compiled into whichever component links it, so the wheel carries two copies and a C++
application cannot use it at all without building from source.

before:
  _portable_lib.so     contains its own copy of the XNNPACK delegate
  training extension   contains another copy
  your C++ app         cannot use the delegate without building from source

after:
  executorch/lib/libexecutorch_xnnpack_backend.so.1     one delegate
    both extensions and your app link the same one

How you use it

find_package(executorch REQUIRED)
target_link_libraries(my_app PRIVATE executorch::runtime executorch::xnnpack_backend)

One detail worth knowing: a delegate registers itself when its library loads, and nothing in
your program calls into it directly. A normal link would therefore drop it as unused, so the
target carries the linker options that keep it. You do not have to do anything for that to
work, but it is why linking the target is enough and no explicit registration call is needed.

Tested

Built the wheel on Linux x86_64 and aarch64, installed it into a clean environment, and
confirmed by symbol inspection that exactly one shipped library defines the delegate. Linked a
C++ application against the component, confirmed the delegate survives the link and registers
itself, and ran a delegated model. Existing Python tests pass unchanged.

[shoumikhin]

Stack from ghstack (oldest at bottom):

• Check the shipped device code covers the GPUs a wheel claims #21567
• Publish CUDA wheels so a GPU user does not build from source #21569
• Ship the CUDA delegate as its own library #21531
• -> Ship the XNNPACK delegate as its own library #21526
• Ship the merged CPU kernels as their own library #21524
• Ship the thread pool as its own library so a process has one pool #21522
• Ship a linkable shared runtime and make the Python bindings use it #21514

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21526

• 📄 Preview Python docs built from this PR

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❌ 21 New Failures, 242 Pending, 1 Unrelated Failure

As of commit 78518c9 with merge base 6bbb75a ():

NEW FAILURES - The following jobs have failed:

• Build Presets / windows (pybind) / build (gh)
• Build Presets / windows (windows) / build (gh)
• pull / test-arm-backend-public-api-backward-compatibility / linux-job (gh)
  RuntimeError: Command docker exec -t 219f6ca46207bf64ade036acedd5e16ada9d14d8ce04d90a66812f306f9d9a8a /exec failed with exit code 1
• pull / test-lora-linux / linux-job (gh)
  AttributeError: '_OpNamespace' 'llama' object has no attribute 'sdpa_with_kv_cache'
• pull / test-lora-multimethod-linux / linux-job (gh)
  AttributeError: '_OpNamespace' 'llama' object has no attribute 'sdpa_with_kv_cache'
• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 11adfeb99da1782e6824713f7d1f517245c09785dbc21a5c46dd18315a330326 /exec failed with exit code 1
• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  AttributeError: '_OpNamespace' 'llama' object has no attribute 'sdpa_with_kv_cache'
• pull / unittest / linux / linux-job (gh)
  backends/xnnpack/test/passes/test_channels_last_tagged_reshape.py::TestChannelsLastTaggedReshapePass::test_conv_linear_dim_order_swaps
• pull / unittest-editable / linux / linux-job (gh)
  backends/xnnpack/test/passes/test_channels_last_tagged_reshape.py::TestChannelsLastTaggedReshapePass::test_conv_linear_dim_order_swaps
• trunk / test-arm-backend-ethos-u (test_deit_e2e_ethos_u) / linux-job (gh)
  RuntimeError: Command docker exec -t b43641fbcd7f7cefd5583b514e3089edba089aa9c4013b5d67cfdcb4215f7d2c /exec failed with exit code 1
• trunk / test-arm-backend-ethos-u (test_model_smollm2_135M_ethos_u85) / linux-job (gh)
  RuntimeError: Command docker exec -t ff8d5b5623d516d78ec28489204f94f07d196b25f197118dcbff263b8760a695 /exec failed with exit code 1
• trunk / test-arm-backend-ethos-u (test_ootb_tests_ethos_u) / linux-job (gh)
  RuntimeError: Command docker exec -t bd329ad979ac73ea31f363f66bc2ad183547445fa0e3fb4d2a1f8b1e5aca9b29 /exec failed with exit code 1
• trunk / test-arm-backend-ethos-u (test_ootb_tests_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 6f6068df86d53efdd21947837f4a5ddbb40c26da0903a34dd19d11d3e1bf3ebc /exec failed with exit code 1
• trunk / test-arm-backend-ethos-u (test_pytest_models_ethos_u85) / linux-job (gh)
  RuntimeError: Command docker exec -t dd470b4e15753327a9ceaa8eaf27fc75972217776c33877fc4d402928c6776ae /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (gemma3-1b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t 8f20ea969022ddb3216dff60c131f0a6ecfe8e8982e792e7240639dddcfc393c /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (llama3.2-1b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t e69b4357c0ea5ba92252cc19fe216ec65829534809e92cf75cf4f81b38ab1ae6 /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (qwen3-0.6b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t 382ba9125bcb06a6f857f9b6e2fa773f39191891383249cc7fb02bc176032166 /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (qwen3-1.7b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t b5b90f932b19e275d0cc750a3e371ab4dcb2267780fded64e65dff4c90823a56 /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (smollm2-135m|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t 9572bb53ee8db0f26fa8dd3f527ed0df8ba174e681b5e183fd923babcb033307 /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (smollm3-3b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t 392090e05bf027e42f1397b46e0b2ade1aa5f170562679e6e757592772b766bc /exec failed with exit code 1
• trunk / unittest-release / linux / linux-job (gh)
  backends/xnnpack/test/passes/test_channels_last_tagged_reshape.py::TestChannelsLastTaggedReshapePass::test_conv_linear_dim_order_swaps

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / windows / windows-job (gh) (detected as infra flaky with no log or failing log classifier)

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