Rearrange export input after checks

[glenn-jocher]

img size checks are warnings (with autocorrection) rather than errors, so current implementation allows improperly formed model inputs.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Refinement of input tensor initialization in models/export.py.

📊 Key Changes

• Input tensor img creation moved after model load and img_size verification.
• Removal of redundant tuple unpacking in input tensor shape.

🎯 Purpose & Impact

• Ensures that the input tensor matches the model's expected input size, increasing robustness.
• Simplifies the code for readability and potential performance benefits.
• Users can expect more reliable model export functionality with updated input size checks. 🚀

[glenn-jocher]

CI passing, merging.