remove deprecated compute_on_step from torchmetrics/image

[phaseolud]

What does this PR do?

Part of #956 - image

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

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[codecov]

Codecov Report

Merging #979 (3e3224a) into master (f4afaf5) will increase coverage by 12%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #979    +/-   ##
======================================
+ Coverage      71%    83%   +12%     
======================================
  Files         177    177            
  Lines        7518   7518            
======================================
+ Hits         5323   6260   +937     
+ Misses       2195   1258   -937     

[phaseolud]

Do I need to update the changelog as well?

[phaseolud]

Codecov Report

Merging #979 (775395b) into master (f8ef656) will decrease coverage by 12%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #979    +/-   ##
======================================
- Coverage      95%    83%   -12%     
======================================
  Files         177    177            
  Lines        7513   7513            
======================================
- Hits         7139   6255   -884     
- Misses        374   1258   +884     

Maybe a simple question, but I'm trying to learn 😄: why does the coverage go down with this commit?

[SkafteNicki]

@phaseolud yes please add to this section:
https://github.com/PyTorchLightning/metrics/blob/a5f2e039f2d90402b402d8f18bfe8808dc6c75c6/CHANGELOG.md?plain=1#L36-L43