Add Ethos-U to LLM extension

[Sebastian-Larsson]

cc @digantdesai @SS-JIA @freddan80 @per @zingo @oscarandersson8218 @mansnils @robell @larryliu0820 @mergennachin @cccclai @helunwencser @jackzhxng

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17700

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❌ 5 New Failures, 1 Unrelated Failure

As of commit 1ab1471 with merge base 9343ec2 ():

NEW FAILURES - The following jobs have failed:

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh)
  RuntimeError: Command docker exec -t 804ba9ee69efb88fde595f6606905dc5405319887dc3c4e198583e35bf82c6a7 /exec failed with exit code 9
• pull / test-llama-runner-qnn-linux (fp32, qnn_8a8w, qnn) / linux-job (gh)
  RuntimeError: Command docker exec -t 8efc7c566175220fc43630941c14fd6312ebadccbcbd8ed64863663a25f426d9 /exec failed with exit code 9
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 49f12daacaa7395e16aed6991ef33ea6e43e6cae2f3b4af5c9890c00f0777f29 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 09f76cc6aa38a24a003f6fe0e58dc3b10db9064f18ae558d9697ab9488dd50bc /exec failed with exit code 1
• trunk / test-llama-runner-qnn-linux (fp32, qnn_8a8w, qnn) / linux-job (gh)
  RuntimeError: Command docker exec -t a9b38161fd616321fd2293e16cebd4ee135e4d0d6cfcd2a90e9281adb7047dcd /exec failed with exit code 9

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

• Test CUDA Builds / test-models-cuda (sdpa) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

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[zingo]

Hi @SS-JIA this is outside Arm stuff :)