Fix multiple errors in the _final_aggregation function for PearsonCorrCoef

[SkafteNicki]

What does this PR do?

Fixes #2893

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• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2980.org.readthedocs.build/en/2980/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36%. Comparing base (5f675f4) to head (c40b68a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2980     +/-   ##
========================================
- Coverage      70%     36%    -34%     
========================================
  Files         350     350             
  Lines       19539   19532      -7     
========================================
- Hits        13668    7027   -6641     
- Misses       5871   12505   +6634