Check TensorBoard logger before adding graph

[JarnoRFB]

Otherwise, an error is thrown if the tensorboard logger is not included.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhancement to logging logic to prevent unnecessary warnings when using TensorBoard without sync_bn.

📊 Key Changes

• Added a check to ensure Tensorboard (self.tb) is active before processing graph additions.
• Maintains the previous condition to skip the graph addition step when sync_bn (synchronized batch normalization) is enabled.

🎯 Purpose & Impact

• The change aims to reduce unnecessary warnings about JIT trace issues when TensorBoard is not in use.
• Users will experience cleaner logs and avoid potential confusion caused by irrelevant warnings. This update should have no impact on model training or inference behavior, but it will improve the user experience for developers monitoring training progress with TensorBoard. 📈✨

[github-actions]

👋 Hello @JarnoRFB, thank you for submitting a YOLOv5 🚀 PR! To allow your work to be integrated as seamlessly as possible, we advise you to:

• ✅ Verify your PR is up-to-date with upstream/master. If your PR is behind upstream/master an automatic GitHub Actions merge may be attempted by writing /rebase in a new comment, or by running the following code, replacing 'feature' with the name of your local branch:

git remote add upstream https://github.com/ultralytics/yolov5.git
git fetch upstream
# git checkout feature  # <--- replace 'feature' with local branch name
git merge upstream/master
git push -u origin -f

• ✅ Verify all Continuous Integration (CI) checks are passing.
• ✅ Reduce changes to the absolute minimum required for your bug fix or feature addition. "It is not daily increase but daily decrease, hack away the unessential. The closer to the source, the less wastage there is." -Bruce Lee

[glenn-jocher]

@JarnoRFB PR is merged. Thank you for your contributions to YOLOv5 🚀 and Vision AI ⭐