Arm backend: Allow Arm executor_runner CMake to run standalone

[usamahz]

Summary

This PR modernizes the ExecuTorch Arm bare-metal runner workflow so users can move from a PyTorch model to a runnable
Arm executor runner with fewer manual build-system steps, stronger validation, and faster repeated local iteration.

The main change is a new standalone Arm executor runner CMake entry point. run.sh now acts as the orchestration
layer for common Ethos-U bare-metal flows: it can derive build directories, configure the standalone runner with Arm
bare-metal defaults, stage generated PTE/BPTE files, validate reused CMake caches, build the needed runner target,
locate the runner binary, and invoke FVP.

Problem

Before this change, the Arm runner workflow depended on manually stitching together ExecuTorch build/install
artifacts, runner CMake configuration, PTE input wiring, toolchain and target settings, optional debug features, and
repeated install/export steps.

That made the workflow harder to explain, fragile in CI, slower to iterate on locally, and easy to break when reusing
a build directory configured for a different target or feature set.

And a shorter version if the PR description is already long:

CMake Architecture Change

flowchart LR
    subgraph Before
        A1["Build ExecuTorch<br/>arm-baremetal preset"] --> A2["Install/export artifacts"]
        A2 --> A3["Configure runner CMake<br/>examples/arm/executor_runner"]
        A4["PTE / BPTE"] --> A3
        A3 --> A5["arm_executor_runner ELF"]
    end

    subgraph After
        B1["run.sh"] --> B2["Validate / choose build dir"]
        B2 --> B3["Standalone runner CMake<br/>examples/arm/executor_runner/standalone"]
        B4["PTE / BPTE"] --> B1
        B3 --> B5["ExecuTorch top-level CMake<br/>as subdirectory"]
        B3 --> B6["Arm CMake helpers + presets"]
        B5 --> B7["arm_executor_runner ELF"]
        B6 --> B7
    end

Loading

What Changed

• Added examples/arm/executor_runner/standalone as the supported standalone CMake entry point for
  arm_executor_runner.
• Added shared Arm CMake helpers for Ethos-U SDK setup, required target validation, and predictable runner output
  paths.
• Updated build_executor_runner.sh and run.sh to use the standalone runner workflow.
• Added deterministic default build directories under --et_build_root.
• Added cache validation for reused build directories, including target, toolchain, selected ops, PTE placement,
  BundleIO, ETDump, and devtools settings.
• Added PTE/BPTE staging so repeated runs can reuse the same configured CMake build directory.
• Integrated selective-op handling into the standalone runner path.
• Cleaned up bare-metal install/export behavior so standalone builds can consume reusable build-tree artifacts.
• Updated Arm README and notebooks for the new workflow.

Iteration Speed

Repeated local PTE-to-runner iteration is now 8x faster because run.sh can reuse the configured standalone CMake build directory, stage updated PTE/BPTE payloads into the existing cache wiring, and rebuild only the needed runner target instead of repeating the full manual configure/install/export flow.

This is a developer workflow speedup, not a model runtime speedup.

Result

For common Ethos-U bare-metal usage, the user-facing path is now script-owned and repeatable:

1. Run Arm setup.
2. Run examples/arm/run.sh with a model and target.
3. Reuse or inspect the generated build directory under --et_build_root.
4. Iterate by regenerating the PTE/BPTE and rebuilding through the same validated CMake cache.

VGF host flows remain explicit: run.sh requires an existing --build-dir for VGF-style host builds rather than
auto-configuring them as bare-metal runner builds.

Testing

Validated through the Arm backend runner, bare-metal, VGF, and CI workflows covered by this stack.

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

[pytorch-bot]

🔗 Helpful Links

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• pull / unittest / macos / macos-job (gh)
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• pull / unittest / windows / windows-job (gh)
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• pull / unittest-arm-backend-with-no-deps (test_run_tosa) / linux-job (gh)
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• pull / unittest-editable / macos / macos-job (gh)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph
• pull / unittest-editable / windows / windows-job (gh)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph
• trunk / test-huggingface-transformers-xnnpack (gemma3-1b|xnnpack|--quantize) / linux-job (gh)
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• trunk / unittest-release / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv2_model
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This comment was automatically generated by Dr. CI and updates every 15 minutes.

[usamahz]

I will merge after all 8 commits land in this PR - so until then please do not merge! :)

[usamahz]

This PR is ready to review

[zingo]

Hi @digantdesai this is large and a lot of cmake changes and replaces a PR you reviewed yesterday in a better way)is OK to merge this?
We don't thing it should need any buck2 changes.

Adding @GregoryComer also, as you where added the the old PR .

[GregoryComer]

Looks reasonable to me. I'll defer to @digantdesai, but I believe there should be no buck impact and you should be clear to merge.