Add --gist flag to upload tlparse output to GitHub Gist

[ezyang]

Summary

• Adds --gist CLI flag to upload tlparse output to a secret GitHub Gist
• Checks for gh CLI availability and authentication before uploading
• Shows an interactive privacy warning explaining that the output contains model architecture details and that secret gists are accessible to anyone with the link
• Requires explicit y/N confirmation before uploading

Motivation

Open source contributors are often confused about what artifacts to send when asked for tlparse output. This feature allows them to easily upload and share their tlparse artifacts via GitHub Gist, making it simpler to collaborate with PyTorch developers.

Usage

tlparse /path/to/torch_trace.log --gist

The user will see a privacy warning and must confirm before upload:

╔════════════════════════════════════════════════════════════════════╗
║                        ⚠️  PRIVACY WARNING ⚠️                        ║
╠════════════════════════════════════════════════════════════════════╣
║ The tlparse output contains detailed information about your model, ║
║ including:                                                         ║
║   • Model architecture and structure                               ║
║   • Graph operations and transformations                           ║
║   • Compilation traces and debug information                       ║
║                                                                    ║
║ This gist will be SECRET but ANYONE WITH THE LINK can view it.    ║
║ GitHub secret gists are not truly private - they are unlisted     ║
║ but accessible to anyone who has the URL.                         ║
╚════════════════════════════════════════════════════════════════════╝

Do you want to proceed with uploading to GitHub Gist? [y/N]: 

Test plan

• Run tlparse <log> --gist without gh CLI installed - should show helpful error
• Run tlparse <log> --gist without gh authentication - should prompt to authenticate
• Run tlparse <log> --gist and type n - should cancel upload
• Run tlparse <log> --gist and type y - should upload and display gist URL

Fixes #161