Update check_file() avoid repeat URL downloads

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improved file download logic with local file existence check.

📊 Key Changes

• Added a check to see if a file already exists locally before attempting to download it from a URL.
• If the file exists, it logs a message stating that the file was found locally.
• If the file does not exist, it proceeds with the download and logs the download attempt.

🎯 Purpose & Impact

• Reduce Unnecessary Downloads: Prevents unnecessary network traffic and reduces download time by using existing files.
• Feedback to Users: Provides clear messages to users about whether a file was found locally or downloaded.
• Ensure File Integrity: Maintains a check to ensure that after a file download, it exists and has content to prevent errors from an incomplete download.