Add missing model tracing step to cpp example readme, fixes #7952

[abstractionmage]

When you follow the instructions for building and running the c++ hello_world example project, you will get the message error loading the model printed to the console.

This is because trace_model.py is never called, so the model file resnet18.pt is never created and also never copied over to the build folder where the compiled binary expects it to be.

In issue #7952, pmeier suggested that the tracing step shoud be added to the example projec's readme.md, which this PR does.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/7963

• 📄 Preview Python docs built from this PR

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❌ 22 New Failures, 10 Unrelated Failures

As of commit 9e61220 with merge base b6574c9 ():

NEW FAILURES - The following jobs have failed:

• build / linux-job (gh)
• c-source / linux-job (gh)
• linux (linux.12xlarge, cpu) / linux-job (gh)
• linux (linux.g5.4xlarge.nvidia.gpu, cuda, 11.8) / linux-job (gh)
• onnx / linux-job (gh)
• python-source-and-configs / linux-job (gh)
• python-types / linux-job (gh)
• unittests-extended / linux-job (gh)
• unittests-linux (3.10, linux.12xlarge, cpu) / linux-job (gh)
• unittests-linux (3.11, linux.12xlarge, cpu) / linux-job (gh)
• unittests-linux (3.8, linux.12xlarge, cpu) / linux-job (gh)
• unittests-linux (3.8, linux.g5.4xlarge.nvidia.gpu, cuda, 11.8) / linux-job (gh)
• unittests-linux (3.9, linux.12xlarge, cpu) / linux-job (gh)
• unittests-macos (3.11, macos-12) / macos-job (gh)
• unittests-prototype (3.10, linux.12xlarge, cpu) / linux-job (gh)
• unittests-prototype (3.11, linux.12xlarge, cpu) / linux-job (gh)
• unittests-prototype (3.8, linux.12xlarge, cpu) / linux-job (gh)
• unittests-prototype (3.8, linux.g5.4xlarge.nvidia.gpu, cuda, 11.8) / linux-job (gh)
• unittests-prototype (3.9, linux.12xlarge, cpu) / linux-job (gh)
• unittests-windows (3.10, windows.4xlarge, cpu) / windows-job (gh)
• unittests-windows (3.8, windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job (gh)
• windows (windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job (gh)

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• macos (macos-12) / macos-job (gh)
• macos (macos-m1-12) / macos-job (gh)
• unittests-macos (3.10, macos-12) / macos-job (gh)
• unittests-macos (3.8, macos-12) / macos-job (gh)
• unittests-macos (3.8, macos-m1-12) / macos-job (gh)
• unittests-macos (3.9, macos-12) / macos-job (gh)
• unittests-windows (3.11, windows.4xlarge, cpu) / windows-job (gh)
• unittests-windows (3.8, windows.4xlarge, cpu) / windows-job (gh)
• unittests-windows (3.9, windows.4xlarge, cpu) / windows-job (gh)
• windows (windows.4xlarge, cpu) / windows-job (gh)

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[pmeier]

Thanks @abstractionmage!

[github-actions]

Hey @pmeier!

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