Add load checkpoint callback

[irenedea]

Depends on mosaicml/composer#3641

[dakinggg]

request a test that the callback can be instantiated (will also force this PR to wait until the next composer release)

[irenedea]

@dakinggg do we do this for other callbacks? Shouldn't instantiation be covered by whether or not I can import it (which it's failing until the composer release is out) and registry tests?

[dakinggg]

@irenedea I don't think we've been super consistent (and you're right that the import covers what I said), but I tend try to write a simple instantiation test for things too (e.g.

llm-foundry/tests/callbacks/test_system_metrics_monitor.py

Line 9 in 788c1f5

def test_system_metrics_monitor_callback_builds():

). Possibly should just write a single parametrized test that iterates over all the callbacks and tries to instantiate them.

[irenedea]

@dakinggg added generic test for callback building here #1577