[gemini] optimize reduce scatter d2h copy (#5760)

* [gemini] optimize reduce scatter d2h copy

* [fix] fix missing reduce variable

* [refactor] remove legacy async reduce scatter code

* [gemini] missing sync

* Revert "[refactor] remove legacy async reduce scatter code"

This reverts commit 58ad76d4665032bbe548d066116d1c572ce98979.

* [gemini] further optimize with async all reduce

* [fix] pass flag from manager to chunk