allow registration of custom checkpoint conversion mappings

[winglian]

What does this PR do?

For custom modeling, this provides a clean route to adding new or overwriting existing mappings with custom mappings. Additionally, it properly memoizes the cached mappings.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@ArthurZucker
Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

[ArthurZucker]

Very nice! I'll be a bit greedy and aski if you can add a small test! 🤗 in the core model loading test file