chore: bump mypy from 1.15.0 to 1.16.0 and resolve typing issues

[rittik9]

What does this PR do?

Fixes #20870

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📚 Documentation preview 📚: https://pytorch-lightning--20900.org.readthedocs.build/en/20900/

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86%. Comparing base (ab619cd) to head (eb9388c).
⚠️ Report is 203 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20900   +/-   ##
=======================================
  Coverage      86%      86%           
=======================================
  Files         268      268           
  Lines       23442    23442           
=======================================
+ Hits        20261    20270    +9     
+ Misses       3181     3172    -9