Simplify and fix --save-period epoch 0

[glenn-jocher]

Fix now allows for epoch 0 saving

🛠️ PR Summary

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🌟 Summary

Simplified checkpoint saving condition during training in YOLOv5.

📊 Key Changes

• Removed the redundant epoch check (epoch > 0) from the condition that determines when to save a checkpoint during training.

🎯 Purpose & Impact

• 💡 Purpose: Streamlining the code to avoid unnecessary checks which can improve code readability and slight performance.
• 🔍 Impact: Users will not notice a difference in how the model saves checkpoints; however, the codebase itself is minimally cleaner and more efficient. There is no change in functionality, just a refactor of the condition logic.