Fix cache in DNSMOS metric

[SkafteNicki]

What does this PR do?

Fixes #2924

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2974.org.readthedocs.build/en/2974/

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 36%. Comparing base (3c01c57) to head (08fb977).
Report is 3 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (3c01c57) and HEAD (08fb977). Click for more details.

HEAD has 52 uploads less than BASE

Flag	BASE (3c01c57)	HEAD (08fb977)
torch2.0.1+cpu	6	3
python3.10	18	9
Windows	4	2
cpu	27	14
torch2.6.0+cpu	2	1
python3.12	5	3
macOS	5	3
torch2.0.1	4	2
Linux	18	9
torch2.3.1+cpu	2	1
python3.9	2	1
torch2.4.1+cpu	4	2
python3.11	2	1
torch2.2.2+cpu	2	1
torch2.5.0+cpu	4	2
torch2.1.2+cpu	2	1

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2974     +/-   ##
========================================
- Coverage      70%     36%    -34%     
========================================
  Files         348     348             
  Lines       19469   19475      +6     
========================================
- Hits        13607    6977   -6630     
- Misses       5862   12498   +6636