Build QNN backend tests with EXTENSION_TENSOR, mark option as required

[GregoryComer]

The QNN backend references extension_tensor and thus requires it to be built. This PR makes two changes:

• Build extension_tensor in the backend test flow (unbreaks QNN jobs).
• Mark the option as required to build QNN to give a nicer error message.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18140

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 3 New Failures, 284 Pending, 1 Unrelated Failure

As of commit 650f4be with merge base cc27e6b ():

NEW FAILURES - The following jobs have failed:

• Build Presets / windows (pybind) / build (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1
• Build Presets / windows (windows) / build (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1
• Windows MSVC Build / build-windows-msvc / windows-job (gh)

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

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

• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1

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[manuelcandales]

@pytorchbot cherry-pick --onto release/1.2 -c critical

[pytorch-bot]

❌ 🤖 pytorchbot command failed:

@pytorchbot cherry-pick: error: argument -c/--classification: invalid choice: 'testci' (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.

[manuelcandales]

@pytorchbot cherry-pick --onto release/1.2 -c critical

[pytorchbot]

Cherry picking #18140

The cherry pick PR is at #18148 and it is recommended to link a critical cherry pick PR with an issue.

Details for Dev Infra team

Raised by workflow job