feat: Add Nordic AXON NPU backend for nRF54LM20B

[petriok]

Summary

ExecuTorch backend for Nordic Semiconductor's AXON NPU on the nRF54LM20B (ARM Cortex-M33 + hardware neural network accelerator).

Follows the same composition pattern as the Ethos-U backend: reuses TOSABackend for TOSA lowering, then compiles to AXON command
buffers via Nordic's compiler library.

Python backend (backends/nordic/):

• AxonBackend, AxonPartitioner, AxonQuantizer, AxonCompileSpec
• TOSA → AXON compiler bridge with per-op converters
• Subgraph naming (content-hash), marker format, header code generation
• Operator support: FC (2048), Conv2D (16x16), Pool (32x32), Add, Mul, ReLU/ReLU6, sigmoid/tanh/softmax op extensions

C++ runtime (backends/nordic/runtime/):

• Delegate with marker-based multi-subgraph lookup and profiling API
• Single-precision sigmoid/tanh op extension callbacks

Zephyr integration:

• EXECUTORCH_BUILD_NORDIC_AXON Kconfig option
• Auto-link executorch_delegate_axon in CMake

Examples (examples/nordic/):

Example	Model	AXON subgraphs
hello_axon	sin(x) regression	1
multi_layer	Chained FC classifier	1
simple_rnn	RNN with recurrent state	2

Each example includes an export script, Zephyr firmware, and step-by-step README.

Nordic's sdk-edge-ai (containing the AXON compiler library) is an external dependency, not redistributed. Discovered via
SDK_EDGE_AI_PATH environment variable — same pattern as Ethos-U's dependency on Vela.

Test plan

• 48 unit tests pass without Nordic SDK (TOSA lowering, operator constraints, codegen, per-op conversion)
• 6 SDK-dependent tests skip gracefully with clear message
• All three examples hardware-verified on nRF54LM20DK:

hello_axon: sin(1.57) = 0.967, 163 us @ 128 MHz, 1 AXON delegate
multi_layer: 4/4 classes correct, ~210 us, 1 delegate (chained layers)
simple_rnn: 4 RNN steps, ~690 us/step, 2 delegates (tanh breaks chain)

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18863

• 📄 Preview Python docs built from this PR

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

❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• Rolling out OSDC (ARC) runners on pull workflow for PyTorch trunk commits

⚠️ 11 Awaiting Approval

As of commit 99fdc20 with merge base 411ede2 ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

• Build documentation (gh)
• Build Presets (gh)
• Cadence Build & Test (gh)
• Lint (gh)
• periodic (gh)
• pull (gh)
• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test QNN Backend (gh)
• Test Vulkan Backend (gh)
• Test XNNPACK Backend (gh)

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

[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.