Fix issue with using a callable as a learning rate

[kapoor1992]

Closes #19411.

[fchollet]

Thanks for the fix!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.22%. Comparing base (6a9bc4c) to head (c2e07e7).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #19484   +/-   ##
=======================================
  Coverage   76.21%   76.22%           
=======================================
  Files         367      367           
  Lines       41066    41066           
  Branches     8018     8018           
=======================================
+ Hits        31298    31302    +4     
+ Misses       8055     8053    -2     
+ Partials     1713     1711    -2     

Flag	Coverage Δ
keras	76.07% <100.00%> (+<0.01%)	⬆️
keras-jax	60.28% <100.00%> (+<0.01%)	⬆️
keras-numpy	54.20% <100.00%> (+<0.01%)	⬆️
keras-tensorflow	61.56% <100.00%> (+<0.01%)	⬆️
keras-torch	60.37% <100.00%> (+<0.01%)	⬆️

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