What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind task

What does this PR do / why do we need it:
Merge _exec_save_checkpoint into save_checkpoint.Modify and standardize save_checkpoint API.

Which issue(s) this PR fixes:

Fixes #
https://gitee.com/mindspore/dashboard?locate=pr&issue_id=I1SX6U
Special notes for your reviewers: