Fix model.save() for model YAMLs

[Y-T-G]

Closes #18206

MRE

from ultralytics import YOLO, ASSETS
model = YOLO("yolo11n.yaml")
model.save("saved.pt")

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improves model consistency by updating how checkpoint data is managed during training and loading.

📊 Key Changes

• Changed the default value of self.ckpt from None to an empty dictionary {} for better initialization.
• Updated the train method to save both the model and checkpoint data (self.ckpt) after loading weights.

🎯 Purpose & Impact

• 🛠 Improved Code Reliability: Ensures better handling of checkpoint-related data by initializing it as an empty dictionary, reducing potential runtime issues.
• ⚙️ Enhanced Checkpoint Management: Exposes checkpoint data (ckpt) alongside the model post-training, providing users with more access to training-related information.
• 💡 User Convenience: Easier debugging and usage of checkpoint data for advanced users, improving workflow transparency.

This change primarily benefits developers working with checkpoints and custom workflows, while maintaining backward compatibility. 🚀

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From your PR diff and description, it looks like you are addressing a checkpoint-saving issue by ensuring self.ckpt is initialized. 🎉 This is a helpful improvement for robustness! However, if this change fixes a specific bug or issue, it would be great if you could provide a minimum reproducible example (MRE) demonstrating the error this PR resolves. This will help us validate and test your fix more efficiently.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.00%. Comparing base (626e42e) to head (e71ec70).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18212      +/-   ##
==========================================
- Coverage   78.52%   74.00%   -4.53%     
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  Files         128      128              
  Lines       17138    17138              
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- Hits        13458    12683     -775     
- Misses       3680     4455     +775     

Flag	Coverage Δ
Benchmarks	35.03% <50.00%> (ø)
GPU	38.47% <100.00%> (-2.64%)	⬇️
Tests	67.76% <100.00%> (-4.67%)	⬇️

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[Laughing-q]

@Y-T-G looks good! perhaps we could directly initialize self.ckpt={} at the first place instead of None, then I guess we don't need:

self.ckpt = self.ckpt or {}

[Y-T-G]

@Laughing-q Yeah, that sounds better

[glenn-jocher]

@Y-T-G @Laughing-q there's a PEP violation that happens when initializing function arguments to non-immutable types like lists and dicts, but it's really annoying and I think we may want to start ignoring it, as otherwise the official standard is to initialize these to None and then have an if variable == None: then set it to what you originally wanted, i.e. variable = {}.

This is why you'll often see arguments initialized to None and then changed later in the code.

[glenn-jocher]

@Y-T-G merged!