Add bounded concurrent syncing with --max-workers

[pierreadorni]

I often have multiple runs at the same time, and found that one long wandb sync would be blocking the sync of my other (shorter) runs. I therefore added a concurrency feature to allow for syncing multiple runs at the same time.

Summary

• Add bounded concurrent syncing in the watcher to avoid one long wandb sync blocking other runs.
• Expose --max-workers CLI option (default 1, preserves current behavior).
• Document the new option and add a changelog entry.
• Add tests for concurrency scheduling and invalid max_workers.

Possible improvement

The new concurrency kind of messes up the display when multiple wandb sync processes write to stdout at the same time. This could be solved by capturing the output of the wandb sync command and printing our own info in a more concurrency-friendly manner, but I chose to not do it as to limit code changes to the minimum.

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
src/wandb_osh/syncer.py	48% smaller
CHANGELOG.md	Unsupported file format
README.md	Unsupported file format
src/wandb_osh/cli.py	0% smaller
tests/test_syncer.py	0% smaller