Update C++ frontend tutorial with template, working MNIST download link, and newer C++ / CMake versions

[jbschlosser]

T237454564

Description

Updated the tutorial:

• Added template header with clear prerequisites
• Updated C++ version to 17
• Updated CMake to >= 3.5 from https://github.com/pytorch/examples/blob/main/cpp/dcgan/CMakeLists.txt
• Fixed the link in the referenced download_mnist.py script to use the mirror at https://github.com/fgnt/mnist

Checklist

• The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3570

• 📄 Preview Python docs built from this PR

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✅ No Failures

As of commit 5dfeb59 with merge base bafe9d4 ():
💚 Looks good so far! There are no failures yet. 💚

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[jbschlosser]

@svekars are you okay to review this?