Fix BaseOptimizer to handle a mix of tf.Variable and KerasVariable

[james77777778]

Fix #20517

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.12%. Comparing base (e5351f3) to head (84585c4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20534   +/-   ##
=======================================
  Coverage   82.11%   82.12%           
=======================================
  Files         515      515           
  Lines       47802    47809    +7     
  Branches     7488     7490    +2     
=======================================
+ Hits        39254    39261    +7     
  Misses       6736     6736           
  Partials     1812     1812           

Flag	Coverage Δ
keras	81.97% <100.00%> (+<0.01%)	⬆️
keras-jax	65.06% <40.00%> (+<0.01%)	⬆️
keras-numpy	60.03% <40.00%> (+<0.01%)	⬆️
keras-tensorflow	66.04% <100.00%> (+<0.01%)	⬆️
keras-torch	64.98% <40.00%> (+<0.01%)	⬆️

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