Fix ops.stop_gradient for KerasTensor

[alexhartl]

Fixes #20124.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.32%. Comparing base (6fd4873) to head (69e63e8).
Report is 182 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20125      +/-   ##
==========================================
+ Coverage   73.70%   79.32%   +5.61%     
==========================================
  Files         501      501              
  Lines       46988    46990       +2     
  Branches     8663     8664       +1     
==========================================
+ Hits        34632    37273    +2641     
+ Misses      10659     7969    -2690     
- Partials     1697     1748      +51     

Flag	Coverage Δ
keras	79.17% <100.00%> (+5.54%)	⬆️
keras-jax	62.49% <100.00%> (+<0.01%)	⬆️
keras-numpy	57.55% <100.00%> (+<0.01%)	⬆️
keras-tensorflow	63.85% <100.00%> (+<0.01%)	⬆️
keras-torch	62.53% <100.00%> (?)

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[fchollet]

LGTM, thank you