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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❌ 3 New Failures, 1 Unrelated Failure, 1 Unclassified Failure

As of commit 4651e01 with merge base 1f7c6d0 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t ee9b1131754e3472f20988ad244fd6ed841cfb77a33490abd28965deb6f0a38d /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t ace36a080b60a7e3e6b083de3e7b8603e4efa14fd440ce5675b322e727e29baa /exec failed with exit code 1
• trunk / unittest-release / linux / linux-job (gh)
  RuntimeError: Command docker exec -t f9880bbebf2055833e74e6cf7a0a9d003430601da40a2feb9e9d921d0daec6b5 /exec failed with exit code 1

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• pull / test-coreml-bc-macos (macos-m1-stable) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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

[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

[usamahz]

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

[pytorchbot]

Cherry picking #21397

The cherry pick PR is at #21460 The following tracker issues are updated:

• [v1.4.0] Release Schedule and Tracker #20552 (comment)

Details for Dev Infra team

Raised by workflow job