Daemon thread mosaic plots fix

[glenn-jocher]

Partial fix for #1937.

  File "train.py", line 518, in <module>
    train(hyp, opt, device, tb_writer, wandb)
  File "train.py", line 322, in train
    wandb.log({"Mosaics": [wandb.Image(str(x), caption=x.name) for x in save_dir.glob('train*.jpg')]})
  File "train.py", line 322, in <listcomp>
    wandb.log({"Mosaics": [wandb.Image(str(x), caption=x.name) for x in save_dir.glob('train*.jpg')]})
  File "/usr/local/lib/python3.6/dist-packages/wandb/data_types.py", line 1555, in __init__
    self._initialize_from_path(data_or_path)
  File "/usr/local/lib/python3.6/dist-packages/wandb/data_types.py", line 1625, in _initialize_from_path
    self._image = pil_image.open(path)
  File "/usr/local/lib/python3.6/dist-packages/PIL/Image.py", line 2862, in open
    "cannot identify image file %r" % (filename if filename else fp)
PIL.UnidentifiedImageError: cannot identify image file 'runs/train/bat_16/train_batch2.jpg'

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improved WandB logging for training mosaic images in YOLOv5.

📊 Key Changes

• Adjusted condition for logging mosaics from ni == 3 to ni > 3.
• Added a file existence check before logging images to WandB.

🎯 Purpose & Impact

• 🎯 Purpose: Ensure reliable logging of mosaic images to WandB, preventing errors when image files do not exist.
• 💪 Impact: Enhances the robustness of training logs in WandB and prevents potential disruptions due to missing files. Users of YOLOv5 will experience more consistent tracking of their model training progress.

[glenn-jocher]

/rebase