Improve the checkpoint upgrade utility script

[awaelchli]

What does this PR do?

This PR improves the special cmdline script for upgrading checkpoints:

• Now accepts a directory and can support search
• Error messaging when file(s) are not found
• No longer overwrites existing backup files if the running the command twice. Losing a checkpoint like this could be detrimental.
• Now has a progress bar
• Also adds missing docs

Part of #9396

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @awaelchli @ananthsub @ninginthecloud @rohitgr7 @otaj

[tchaton]

Looks neat !