Create callback to load checkpoint

[irenedea]

What does this PR do?

Adds a new callback to load a checkpoint at a specified event. This enables use cases like loading model base weights with peft.

Manual Test

Ran two lora finetuning runs.
test-load-checkpoint-callback-baseline-K0m4fq: Initialized from Huggingface checkpoint and ran for 100 batches
test-load-checkpoint-callback-resume-GS8oWj: Initialized using this callback and the baseline checkpoint at ba50.

We see that the losses match up.

Also confirmed that the adapter-only checkpoint is smaller than a full checkpoint as expected.

What issue(s) does this change relate to?

Before submitting

• Have you read the contributor guidelines?
• Is this change a documentation change or typo fix? If so, skip the rest of this checklist.
• Was this change discussed/approved in a GitHub issue first? It is much more likely to be merged if so.
• Did you update any related docs and document your change?
• Did you update any related tests and add any new tests related to your change? (see testing)
• Did you run the tests locally to make sure they pass?
• Did you run pre-commit on your change? (see the pre-commit section of prerequisites)

[dakinggg]

looks right to me