Add flag to save the final checkpoint as weights only

[irenedea]

What does this PR do?

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)