Do not optimize CoreML TorchScript model

[glenn-jocher]

Fixes CoreML export error introduced in PR 'Default optimize_for_mobile() on TorchScript models' #2908

Mobile-optimized TorchScript model is now saved to disk but not saved locally, and thus is not used later in CoreML export.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Optimization step streamlined in torchscript model export.

📊 Key Changes

• The optimize_for_mobile function's output is now directly saved without storing it in a variable.

🎯 Purpose & Impact

• 👩‍💻 Reduces code complexity, making it simpler and easier to maintain.
• 🔍 Ensures the export process is more succinct and possibly slightly faster.
• 🚀 Users exporting models to TorchScript for mobile deployment now have a neater codebase to work with, with unchanged functionality.