tests: skip if pytest.pool not used

[martin-kokos]

What does this PR do?

Fixes failing test when

tests/unittests/bases/test_ddp.py sssssssssssssssF

=================================================================================================================== FAILURES ===================================================================================================================
______________________________________________________________________________________________________ test_sync_with_unequal_size_lists _______________________________________________________________________________________________________

    @pytest.mark.DDP
    @pytest.mark.skipif(not _TORCH_GREATER_EQUAL_2_1, reason="test only works on newer torch versions")
    @pytest.mark.skipif(sys.platform == "win32", reason="DDP not available on windows")
    def test_sync_with_unequal_size_lists():
        """Test that synchronization of states can be enabled and disabled for compute."""
>       pytest.pool.map(_test_sync_with_unequal_size_lists, range(NUM_PROCESSES))
E       AttributeError: module 'pytest' has no attribute 'pool'


tests/unittests/bases/test_ddp.py:346: AttributeError
=========================================================================================================== short test summary info ============================================================================================================
FAILED tests/unittests/bases/test_ddp.py::test_sync_with_unequal_size_lists - AttributeError: module 'pytest' has no attribute 'pool'

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• [] Did you make sure to update the docs?
• [] Did you write any new necessary tests?

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---

📚 Documentation preview 📚: https://torchmetrics--2930.org.readthedocs.build/en/2930/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36%. Comparing base (520a868) to head (0347709).
Report is 15 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (520a868) and HEAD (0347709). Click for more details.

HEAD has 270 uploads less than BASE

Flag	BASE (520a868)	HEAD (0347709)
torch2.6.0+cpu	10	2
Windows	15	3
python3.11	20	4
cpu	84	17
torch2.0.1+cpu	15	3
python3.10	44	9
torch2.5.0+cpu	15	3
python3.12	15	3
macOS	20	4
torch2.5.0	5	1
torch2.6.0	5	1
torch2.0.1	10	2
Linux	49	10
torch2.4.1+cpu	10	2
torch2.3.1+cpu	4	1
torch2.1.2+cpu	5	1
torch2.2.2+cpu	5	1
python3.9	5	1
gpu	1	0
unittest	1	0

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2930     +/-   ##
========================================
- Coverage      69%     36%    -33%     
========================================
  Files         346     332     -14     
  Lines       19146   18996    -150     
========================================
- Hits        13233    6793   -6440     
- Misses       5913   12203   +6290