Fix overflow for PSNR metric when used with uint8 input

[SkafteNicki]

What does this PR do?

Fixes #2787

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--2788.org.readthedocs.build/en/2788/

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 38%. Comparing base (6377aa5) to head (e86c101).
Report is 1 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (6377aa5) and HEAD (e86c101). Click for more details.

HEAD has 240 uploads less than BASE

Flag	BASE (6377aa5)	HEAD (e86c101)
Windows	12	3
python3.8	12	3
cpu	80	20
torch1.13.1+cpu	12	3
macOS	16	4
python3.10	20	5
torch2.0.1	8	2
torch2.4.0+cpu	4	1
python3.11	12	3
torch1.12.1+cpu	4	1
Linux	52	13
python3.9	36	9
torch2.4.0	4	1
torch1.10.2+cpu	4	1
torch2.0.1+cpu	12	3
torch1.11.0+cpu	4	1
torch2.3.1+cpu	8	2
torch1.13.1	4	1
torch2.2.2+cpu	8	2
torch2.1.2+cpu	4	1
torch2.4.0+cu121	4	1

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2788     +/-   ##
========================================
- Coverage      70%     38%    -32%     
========================================
  Files         336     336             
  Lines       18231   18235      +4     
========================================
- Hits        12707    6913   -5794     
- Misses       5524   11322   +5798