fix reference problem

[Lornatang]

fix Cannot find reference 'hardtanh' problem"

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhancement of activation function imports for better code stability.

📊 Key Changes

• Fixed the incorrect import statement of the torch functions.

🎯 Purpose & Impact

• Purpose: To correct a typo in the import statement which could potentially lead to bugs or unexpected behavior when using activation functions.
• Impact: Ensures that the activation functions within the YOLOv5 model work as expected, contributing to model reliability and performance stability. Users will experience more robust code execution with fewer import-related issues. 🛠️

[glenn-jocher]

@Lornatang great, thank you for discovering the issue and submitting this PR!