Improve executor_runner cmake instructions

[guangy10]

Summary

There is a mandatory flag DEXECUTORCH_BUILD_EXECUTOR_RUNNER must be turned on in order to build executor_runner, but this flag is not explicitly set in our tutorial. Though the default of it is ON, it can still be a UX issue when following the tutorial where the flag was toggled OFF silently when building for other target like llama_runner.

Test plan

CI

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11773

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❌ 4 New Failures

As of commit 4163a7d with merge base 7503bb3 ():

NEW FAILURES - The following jobs have failed:

• Build Linux Wheels / pytorch/executorch / build-manywheel-py3_10-cpu (gh)
  ModuleNotFoundError: No module named 'torchvision'
• Build Linux Wheels / pytorch/executorch / upload / upload-manywheel-py3_10-cpu (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_x86_64
• Build macOS Wheels / pytorch/executorch / build-wheel-py3_10-cpu (gh)
  ModuleNotFoundError: No module named 'torchvision'
• Build macOS Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_

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