route EthosU input/output memcpy through overridable hook

[3l1]

Summary:
The EthosU backend's input/output scratch shuffling currently does plain
CPU std::memcpy of every input tensor into the scratch buffer and every
output tensor out of it on every inference. On Cortex-M55-based firmware
targets that have a DMA engine, this is a significant CPU load, inference is spent in memcpy that could instead be DMA-offloaded so
the M55 sleeps while the transfer runs.

This change introduces a thin extern-C indirection — arm_ethos_io_memcpy
— that the EthosU backend uses everywhere it currently calls memcpy for
input/output scratch shuffling. The default (weak) implementation lives
in a separate translation unit (EthosUBackend_IoMemcpy.cpp) and just
calls std::memcpy, so behavior is unchanged for any consumer that doesn't
override it.

Firmware targets can supply a strong-symbol override (e.g. routing
through a DMA engine) without touching the upstream backend code.

Implementation notes:

• The weak default lives in its own TU so the compiler in the call-site
  TUs cannot inline its body and bypass the link-time override. This is
  the same pattern bolt_arm_memcpy_external uses.
• Three call sites updated: input scratch copy in EthosUBackend.cpp, the
  layout-adjustment chunk loop in EthosUBackend.cpp, and the output
  scratch copy in EthosUBackend_Cortex_M.cpp.

Differential Revision: D103455766

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19264

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 18 New Failures, 3 Unrelated Failures

As of commit ffc9927 with merge base a7e44bf ():

NEW FAILURES - The following jobs have failed:

• pull / test-mcu-cortex-m-backend / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• pull / unittest-editable / windows / windows-job (gh)
  examples/models/test/test_export.py::ExportTest::test_efficient_sam_export_to_executorch
• trunk / test-arm-backend-ethos-u (test_memory_allocation) / linux-job (gh)
  RuntimeError: Command docker exec -t 1ec4c757162b760dc316a5830f56ceb9a4014ff4afcb1598444da66c6371ab5d /exec failed with exit code 2
• trunk / test-arm-backend-ethos-u (test_pytest_models_ethos_u55) / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-arm-backend-ethos-u (test_pytest_models_ethos_u85) / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-arm-backend-ethos-u (test_pytest_ops_ethos_u55) / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-arm-backend-ethos-u (test_pytest_ops_ethos_u85) / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-arm-backend-ethos-u (test_run_ethos_u55) / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-arm-backend-ethos-u (test_run_ethos_u85) / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-arm-backend-zephyr (cortex-m55) / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-arm-backend-zephyr (ethos-u55) / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-arm-backend-zephyr (ethos-u85) / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-arm-ootb-linux (run_deit_e2e_ethos_u) / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-arm-ootb-linux (run_ootb_tests_ethos_u) / linux-job (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-cortex-m-e2e / run (mv2) / mv2 (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-cortex-m-e2e / run (mv3) / mv3 (gh)
  undefined reference to arm_ethos_io_memcpy'`
• trunk / test-models-macos-cpu (mobilebert, xnnpack-quantization-delegation) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-static-llama-ane / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• trunk / unittest-release / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[meta-codesync]

@3l1 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D103455766.

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.