Add checkpoint events to mosaicml logger

[b-chu]

What does this PR do?

Adds metadata logging to mosaicml logger for when checkpoint upload starts when a checkpoint is done loading. The information logged is the timestamp and the training duration of the checkpoint.

Tested in events-test-8ReCjx

What issue(s) does this change relate to?

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[mvpatel2000]

Holding based on discussions

[mvpatel2000]

can you include a manual test to verify this works since I don't htink we can easily unit test this?

Edit: maybe its mockable for testing?

[mvpatel2000]

You need to force flush!