Arm backend: Add training & export scripts for DEiT example

[tom-arm]

• Add example training and export scripts into examples/arm/image_classification_example
• Add README.md documenting usage

Signed-off-by: Tom Allsop tom.allsop@arm.com

Change-Id: Iefbcc3f2adaa214c37c5b80dd14cd50c161d83b1

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16315

• 📄 Preview Python docs built from this PR

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❌ 12 New Failures, 1 Pending, 1 Unrelated Failure

As of commit 7e3c8dd with merge base 3233761 ():

NEW FAILURES - The following jobs have failed:

• Apple / build-benchmark-app / macos-job (gh)
• Apple / build-demo-ios / macos-job (gh)
• Build Presets / apple (ios-simulator) / build (gh)
  The process '/opt/homebrew/bin/git' failed with exit code 128
• Build Presets / windows (windows) / build (gh)
• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  RuntimeError: Command docker exec -t d6237233ac4a6bbb7ae7b3dc53dd5a8ac4d74b9c5fb75508ab4cac75480ef6a4 /exec failed with exit code 1
• trunk / test-arm-backend (test_pytest_models_ethosu_fvp) / linux-job (gh)
  RuntimeError: Command docker exec -t 85611ad698e7a40737130caf55c7bc56a9e8b08f020f369329e9f285052fb7f3 /exec failed with exit code 1
• trunk / test-coreml-delegate / macos-job (gh)
  The process '/opt/homebrew/bin/git' failed with exit code 128
• trunk / test-huggingface-transformers-macos (bert|coreml_fp32_gpu|--quantize) / macos-job (gh)
  The process '/opt/homebrew/bin/git' failed with exit code 1
• trunk / test-huggingface-transformers-macos (distilbert|coreml_fp32_gpu|--quantize) / macos-job (gh)
  The process '/opt/homebrew/bin/git' failed with exit code 128
• trunk / test-huggingface-transformers-macos (olmo-1b|coreml_fp32_gpu|--quantize) / macos-job (gh)
  Operation timed out (api.github.com:443)
• trunk / test-huggingface-transformers-macos (qwen3-0.6b|coreml_fp32_gpu|--quantize) / macos-job (gh)
  The process '/opt/homebrew/bin/git' failed with exit code 128
• trunk / test-multimodal-macos (gemma3-4b) / macos-job (gh)
  The process '/opt/homebrew/bin/git' failed with exit code 128

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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[zingo]

Arm: test_T5ForConditionalGeneration_tosa_* fails are unrelated and fails without the PR
The rest of the fails are also unrelated.