Fix for https://github.com/keras-team/keras/issues/20425

[hertschuh]

The issue was caused by the fact that the iterator was not fully consumed and on_epoch_end was not called.

Added an exception to catch this situation in the future.

Added a unit test to test model.fit() with all the combinations of data adapters.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 82.01%. Comparing base (272bb90) to head (812bd8f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...s/src/trainers/data_adapters/py_dataset_adapter.py	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20453   +/-   ##
=======================================
  Coverage   82.01%   82.01%           
=======================================
  Files         514      515    +1     
  Lines       47239    47275   +36     
  Branches     7413     7422    +9     
=======================================
+ Hits        38741    38772   +31     
- Misses       6704     6706    +2     
- Partials     1794     1797    +3     

Flag	Coverage Δ
keras	81.86% <62.50%> (+<0.01%)	⬆️
keras-jax	64.91% <62.50%> (+<0.01%)	⬆️
keras-numpy	59.84% <50.00%> (-0.03%)	⬇️
keras-tensorflow	65.91% <62.50%> (+<0.01%)	⬆️
keras-torch	64.83% <62.50%> (-0.02%)	⬇️

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[fchollet]

LGTM -- thank you for the fix!