Add a BEFORE_LOAD event

[snarayan21]

What does this PR do?

Adds a BEFORE_LOAD event that callbacks can use, that happens right before the checkpoint gets loaded. This more easily enables different functions like saving checkpoints from a fully initialized model + optimizer immediately, or changing the dataset state once the dataloader has been passed to the State object.

What issue(s) does this change relate to?

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[dakinggg]

lgtm, will leave approval for mihir