Allow EarlyStopping to be reused between multiple fits.

[hertschuh]

All values were already reset properly in on_train_begin except best.

Fixes #20521

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.12%. Comparing base (a93828a) to head (977e080).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20533   +/-   ##
=======================================
  Coverage   82.11%   82.12%           
=======================================
  Files         515      515           
  Lines       47806    47808    +2     
  Branches     7490     7491    +1     
=======================================
+ Hits        39258    39260    +2     
  Misses       6736     6736           
  Partials     1812     1812           

Flag	Coverage Δ
keras	81.97% <100.00%> (+<0.01%)	⬆️
keras-jax	65.06% <100.00%> (+<0.01%)	⬆️
keras-numpy	60.04% <100.00%> (+<0.01%)	⬆️
keras-tensorflow	66.04% <100.00%> (+<0.01%)	⬆️
keras-torch	64.98% <100.00%> (+<0.01%)	⬆️

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[fchollet]

LGTM, thanks for the fix.