Add fit start to mosaicmllogger

[ethanma-db]

What does this PR do?

Goal is to add new TRAIN_START run event outlined in this doc:
https://docs.google.com/document/d/1Rk43aTNFXIUULs0OvTMQGXrV8bJkITmjICyGmMrF2Zc/edit

This will correspond to the FIT_START composer event

What issue(s) does this change relate to?

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