[bugfix] PSNR should use add_state

[tchaton]

Before submitting

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

What does this PR do?

Fixes Lightning-AI/pytorch-lightning#7273, Lightning-AI/pytorch-lightning#7257

PR review

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[codecov]

Codecov Report

Merging #214 (b12b4de) into master (8eb462f) will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
+ Coverage   79.66%   79.67%   +0.01%     
==========================================
  Files          90       90              
  Lines        2872     2874       +2     
==========================================
+ Hits         2288     2290       +2     
  Misses        584      584              

Flag	Coverage Δ
Linux	79.67% <66.66%> (+0.01%)	⬆️
Windows	79.67% <66.66%> (+0.01%)	⬆️
cpu	79.67% <66.66%> (+0.01%)	⬆️
macOS	79.67% <66.66%> (+0.01%)	⬆️
pytest	79.67% <66.66%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
torchmetrics/regression/psnr.py	78.26% <0.00%> (ø)
torchmetrics/metric.py	55.79% <100.00%> (+0.32%)	⬆️

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[SkafteNicki]

LGTM