Add save and restore config tests to address TODO items.

[naoitoi]

For my first commit to Keras, I picked a simple one, addressing two small TODO items in reduction_metrics_test.py.

I believe what the comment "# TODO: Check save and restore config" meant is that an object (Sum or Mean) must be re-constructed from config, and be tested.

Please let me know if I misunderstood the requirements, or otherwise made any mistakes.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (72cd8be) 73.00% compared to head (4be8a3f) 79.21%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #18812      +/-   ##
==========================================
+ Coverage   73.00%   79.21%   +6.20%     
==========================================
  Files         336      336              
  Lines       34372    34374       +2     
  Branches     6723     6727       +4     
==========================================
+ Hits        25094    27228    +2134     
+ Misses       7740     5575    -2165     
- Partials     1538     1571      +33     

Flag	Coverage Δ
keras	79.07% <ø> (+6.16%)	⬆️
keras-jax	61.55% <ø> (-0.01%)	⬇️
keras-numpy	56.16% <ø> (-0.01%)	⬇️
keras-tensorflow	63.41% <ø> (?)
keras-torch	64.21% <ø> (-0.01%)	⬇️

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[fchollet]

Thanks for the PR! Please run sh shell/format.sh to format the code.

[naoitoi]

Thanks for the PR! Please run sh shell/format.sh to format the code.

Thank you for the quick reply, @fchollet ! I ran format.sh (which deleted one empty line).
Please LMK if there're more things to fix.

Thank you,

Nao

[fchollet]

LGTM, thank you