Remove Redundant Host & Device Synchronizations

[alpha0422]

What does this PR do?

Fixes #21232.

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cc @csarofeen @ptrblck @eqy

[alpha0422]

Can we please add one or more unittests for this? Mostly to check that we do not regress if changes are made to this part of the codebase?

Good point, added a few unittests, thanks.

[alpha0422]

Hi @SkafteNicki, what should I do to move forward?

[SkafteNicki]

@alpha0422 PR looks fine to me. We are having progrems with our CI at the moment so it may take a bit of time before the PR is merged.

[anhnguyen-namitech]

Hope this PR will be merged. We apply this patch, and it speeds up our training a bit.

[SkafteNicki]

@anhnami we are doing our best, the hope is #21309 will fix some of the remaining issues that been blocking a lot of PRs the last couple of weeks

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 79%. Comparing base (a7cb33a) to head (79a26d3). :warning: Report is 6 commits behind head on master. :white_check_mark: All tests successful. No failed tests found.

:exclamation: There is a different number of reports uploaded between BASE (a7cb33a) and HEAD (79a26d3). Click for more details.

HEAD has 103 uploads less than BASE

Flag	BASE (a7cb33a)	HEAD (79a26d3)
cpu	52	27
python	6	3
lightning_fabric	12	0
pytest	27	0
python3.10	12	6
lightning	29	15
python3.11	12	6
python3.12.7	16	9
python3.12	6	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21233     +/-   ##
=========================================
- Coverage      87%      79%     -8%     
=========================================
  Files         269      266      -3     
  Lines       23708    23657     -51     
=========================================
- Hits        20536    18642   -1894     
- Misses       3172     5015   +1843     

[alpha0422]

Hi @SkafteNicki, just wanted to follow up on this PR. Is there anything I can do to help move this forward? I'm happy to make any changes if needed.