feat(config): add checkpoint_fraction into config

Motivation

We have checkpoint_fraction but have no interface in config file, this PR will support this.

Modification

1. add checkpoint_fraction option into model config.
2. add checkpoint_fraction sanity check in launch.py

Checklist

Before PR:

• [x] Pre-commit or other linting tools are used to fix the potential lint issues.
• [x] Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• [x] The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• [ ] The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• [ ] If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects.
• [ ] CLA has been signed and all committers have signed the CLA in this PR.