Update hubconf.py for unified loading

[glenn-jocher]

This improves hubconf.py simplicity by creating a single channel for all tasks. All model loading now passes through one single create() function.

As a plus this also avoids building models when loading locally, which should save time and reduce verbosity for that workflow.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Refinement of model creation and loading functions in Ultralytics YOLOv5.

📊 Key Changes

• Defaulted channels to 3 and classes to 80 in the create function to align with typical YOLOv5 usage.
• Ensured autoshape defaults to True and verbose to True in the create function.
• Updated fname to be a Path object, improving file path handling.
• Simplified the custom model function by utilizing the create function for loading models.

🎯 Purpose & Impact

• 📈 Streamlines the model creation process, reducing the risk of incorrect usage by providing sensible defaults.
• 🛠️ Enhances file path manipulation by using the Path object for better cross-platform support.
• 🔥 Removes redundant code, making it easier to maintain and understand the codebase, which could lead to fewer bugs.
• 🌐 Simplifies the process for users to load custom models using the same interface as predefined models, making the library more user-friendly.

[glenn-jocher]

@kalenmike this should improve our on-demand results now slightly.