Fix TypeError in configure_optimizers when running with ReduceLROnPlateau

[LukasSalchow]

What does this PR do?

Fix 20462

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📚 Documentation preview 📚: https://pytorch-lightning--20471.org.readthedocs.build/en/20471/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83%. Comparing base (de7c28a) to head (3260675).
Report is 42 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20471     +/-   ##
=========================================
- Coverage      88%      83%     -5%     
=========================================
  Files         267      267             
  Lines       23284    23287      +3     
=========================================
- Hits        20387    19314   -1073     
- Misses       2897     3973   +1076     

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[lantiga]

Looks good!

[lantiga]

Merging, this needs further changes but I'll do this in a quick follow-up PR