Add atomic save to checkpoint routine

[corwinjoy]

What does this PR do?

Upgrade the _atomic_checkpoint routine to use a transaction. This should make saves atomic so that if an interrupt happens during a file save, any existing file will not be corrupted.

The following links the related issue to the PR: #19970

Fixes #19970

I have skipped updating the changelog since it looks like this gets updated as part of the release.

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📚 Documentation preview 📚: https://pytorch-lightning--20011.org.readthedocs.build/en/20011/

[awaelchli]

Looks great. Just a few suggestions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59%. Comparing base (3f69134) to head (765e1d1).
Report is 76 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (3f69134) and HEAD (765e1d1). Click for more details.

HEAD has 69 uploads less than BASE

Flag	BASE (3f69134)	HEAD (765e1d1)
python3.10	19	16
cpu	65	48
lightning	39	32
pytest	47	28
examples	9	0
app	9	0
tpu	2	0
lightning_app	3	0

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20011     +/-   ##
=========================================
- Coverage      84%      59%    -25%     
=========================================
  Files         427      422      -5     
  Lines       35504    35407     -97     
=========================================
- Hits        29781    20773   -9008     
- Misses       5723    14634   +8911     

[awaelchli]

Thanks @corwinjoy
I also made a follow-up issue for the distributed checkpoint saving logic.

[corwinjoy]

Thanks for the help @awaelchli !

…

On Tue, Jun 25, 2024, 1:53 AM awaelchli ***@***.***> wrote: Thanks @corwinjoy <https://github.com/corwinjoy> I also made a follow-up issue for the distributed checkpoint saving logic. — Reply to this email directly, view it on GitHub <#20011 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ADBF42XZMLPG43N4Z5Z4RR3ZJEVZFAVCNFSM6AAAAABJ2XXZXKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDCOBYGM2DINZVGM> . You are receiving this because you were mentioned.Message ID: ***@***.***>