Async callback: Don't skip checkpoints, reliably only launch async eval when the checkpoint is ready (#813)
* working without sharded checkpointing..
* add more debugs
* try this
* more debugging
* yikes dumb bug
* add notes
* fixes
* remove prints
* small updates
* fix typo
* refactor
* fix docstring formatting
* fighting with docstrings
* try this
* add unit tests
* point to composer update
* values -> items
* serialize time
* fix merge
* nits
* warning, small comment update
* add error
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Co-authored-by: Daniel King <43149077+dakinggg@users.noreply.github.com>