Arm backend: Add MobileSAM Ethos-U example

[usamahz]

Add a MobileSAM prompt segmentation example for Ethos-U85-256.

Prepare the pinned official source in an external managed checkout.

Apply a small configurable-input patch instead of copying model code.

Use int8 globally and A16W8 for TinyViT attention.

Lower the complete graph to Ethos-U and compare output with FP32.

Emit delegation summaries, validation metrics, and debug artifacts.

Build a bare-metal Corstone-320 app using exporter metadata.

Validate inference on FVP through the Arm OOTB test.

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

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21397

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❌ 1 New Failure, 1 Cancelled Job

As of commit b1885f8 with merge base b3d69a9 ():

NEW FAILURE - The following job has failed:

• Apple / build-frameworks-ios / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-nxp-neutron / linux-job (gh)
  ##[error]The operation was canceled.

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[usamahz]

@pytorchbot cherry-pick --onto release/1.4 -c examples

[pytorch-bot]

❌ 🤖 pytorchbot command failed:

@pytorchbot cherry-pick: error: argument -c/--classification: invalid choice: 'examples' (choose from 'regression', 'critical', 'fixnewfeature', 'docs', 'release')

usage: @pytorchbot cherry-pick --onto ONTO [--fixes FIXES] -c
                               {regression,critical,fixnewfeature,docs,release}

Try @pytorchbot --help for more info.

[usamahz]

@pytorchbot cherry-pick --onto release/1.4 -c release

[usamahz]

@pytorchbot cherry-pick --onto release/1.4 -c docs

[usamahz]

Dont merge, let me check something