Fix functional model output type after deserialization

[shkarupa-alex]

Fix #20119

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.32%. Comparing base (816d219) to head (7c80de5).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20120      +/-   ##
==========================================
- Coverage   79.32%   79.32%   -0.01%     
==========================================
  Files         501      501              
  Lines       46983    46990       +7     
  Branches     8660     8665       +5     
==========================================
+ Hits        37268    37273       +5     
- Misses       7968     7969       +1     
- Partials     1747     1748       +1     

Flag	Coverage Δ
keras	79.17% <100.00%> (-0.01%)	⬇️
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%> (-0.01%)	⬇️

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[fchollet]

Thanks for the PR -- please add a unit test to check this case.

[fchollet]

LGTM, thank you!