Check Result before calling .get() on method_meta in getUsedBackends

[kirklandsign]

Summary

getUsedBackends() called module_->method_meta(...).get() without checking if the Result was ok, which is undefined behavior on error. Add an .ok() check and throw IllegalArgumentException on failure, consistent with how method_names() handles its Result above.

Test plan

CI

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17607

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❌ 4 New Failures, 1 Unrelated Failure

As of commit ffc5e03 with merge base cf08087 ():

NEW FAILURES - The following jobs have failed:

• Build Presets / windows (windows) / build (gh)
• pull / test-mediatek-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t ec281ca5a0bae5115e473ee2927eefc838c0277ca88803c8bf20565106a8863c /exec failed with exit code 2
• pull / test-openvino-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 85e70cbc5acf74274a3d1901e043eda0cedeb3c669b96078a5d84faf1def70ee /exec failed with exit code 1
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t df4746fcf6b8d6f5585952a3a8f35807a79f712c1aed0cf2f10304e6341ccaac /exec failed with exit code 3

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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