Refactor DDP test to use multiprocessing context

[rittik9]

Refactor DDP test to use spawn context for process management.

What does this PR do?

Fixes #<issue_number>

Use mp.get_context('spawn') instead of mp.spawn in tests to prevent module reimport.

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• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--3298.org.readthedocs.build/en/3298/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31%. Comparing base (fd967c0) to head (0847a6c).

❌ Your project check has failed because the head coverage (31%) is below the target coverage (95%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (fd967c0) and HEAD (0847a6c). Click for more details.

HEAD has 680 uploads less than BASE

Flag	BASE (fd967c0)	HEAD (0847a6c)
gpu	2	0
unittest	2	0
Windows	22	2
cpu	186	17
python3.12	44	4
torch2.8.0+cpu	33	3
torch2.0.1+cpu	33	3
python3.10	131	12
torch2.8.0	11	1
macOS	32	3
Linux	132	12
torch2.7.1+cpu	22	2
torch2.4.1+cpu	11	1
torch2.2.2+cpu	11	1
torch2.0.1	21	2
torch2.5.1+cpu	11	1
torch2.1.2+cpu	11	1
torch2.6.0+cpu	11	1
torch2.3.1+cpu	11	1
python3.9	11	1

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #3298     +/-   ##
========================================
- Coverage      37%     31%     -6%     
========================================
  Files         364     349     -15     
  Lines       20096   19901    -195     
========================================
- Hits         7520    6196   -1324     
- Misses      12576   13705   +1129     

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[rittik9]

@Borda @SkafteNicki @justusschock for now can we revert these two prs #3166
#3162 until we fix the test failures in new CI even though they passed in old azure CI?

After reverting we can merge all the pending ready prs.

[Borda]

You can revert it in this PR and continue in fixing and then re-add it also with this PR