Fix error in ergas calculation

[SkafteNicki]

What does this PR do?

Fixes #2497
Fixes #2416

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--2498.org.readthedocs.build/en/2498/

[Borda]

Wondering how the tests are passing before and now with two different computation...

[SkafteNicki]

Wondering how the tests are passing before and now with two different computation...

Because the reference implementation was also written by us (because no reference seems to exist) which had the same computational bug

[codecov]

Codecov Report

Merging #2498 (c580d19) into master (5892316) will decrease coverage by 31%.
Report is 1 commits behind head on master.
The diff coverage is 100%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2498     +/-   ##
========================================
- Coverage      69%     38%    -31%     
========================================
  Files         307     307             
  Lines       17396   17396             
========================================
- Hits        11981    6659   -5322     
- Misses       5415   10737   +5322