Fix ModuleAttributeError in DP and DDP mode

[NanoCode012]

Fixes #682 .

I just made this quick PR to save you time. If you plan to change it somehow else, I can close this PR.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Relocation of image size verification in YOLOv5 training script for enhanced code structure.

📊 Key Changes

• Relocated image size verification code to an earlier position in the training script.
• Ensured image size verification occurs immediately after loading a pre-trained model and before initializing distributed training setup.

🎯 Purpose & Impact

• Refinement of code flow: The verification of image size now happens shortly after any pre-existing model parameters are loaded. This tweak improves the logical flow of operations.
• Potential decrease in bugs or errors: By checking image sizes earlier in the process, issues related to non-conforming image sizes can be caught sooner, possibly reducing errors during training setup.
• No direct impact on model performance: This adjustment is for code quality and does not inherently change model training or performance, meaning users should see no difference in the behavior of their trained models.