Fix async checkpoint timing in DCP recipe

[patrocinio]

Move checkpoint_future.result() before optimizer.step() to ensure the previous checkpoint completes before weights are modified in-place. This allows better overlap of checkpointing with forward/backward passes.

Fixes #3584

Description

Checklist

• [ ] The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• [ ] Only one issue is addressed in this pull request
• [ ] Labels from the issue that this PR is fixing are added to this pull request
• [ ] No unnecessary issues are included into this pull request.

cc @wconstab @osalpekar @H-Huang @kwen2501