Slice serialization

[gleize]

Models that contain slicing cannot be serialized because of the missing handling of slices in the (de)serialize_keras_object functions.

Thus a simple functional as this one.

x = inp = Input(...)
...
x = x[:, 1:-1, 1:-1]
...
fn = Functional(inp, x)

could not be serialized.

Now it's possible with this fix.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +3.99% 🎉

Comparison is base (c64de55) 79.73% compared to head (940163d) 83.73%.
Report is 35 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #932      +/-   ##
==========================================
+ Coverage   79.73%   83.73%   +3.99%     
==========================================
  Files         318      318              
  Lines       28627    28737     +110     
  Branches     5447     5489      +42     
==========================================
+ Hits        22827    24062    +1235     
+ Misses       4333     3159    -1174     
- Partials     1467     1516      +49     

Flag	Coverage Δ
keras_core	83.63% <100.00%> (+3.96%)	⬆️
keras_core-jax	67.07% <100.00%> (?)
keras_core-numpy	60.38% <100.00%> (-0.01%)	⬇️
keras_core-tensorflow	66.87% <100.00%> (+0.05%)	⬆️
keras_core-torch	69.14% <100.00%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
keras_core/ops/numpy.py	94.40% <100.00%> (-0.05%)	⬇️
keras_core/saving/serialization_lib.py	73.77% <100.00%> (+0.37%)	⬆️

... and 44 files with indirect coverage changes

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[fchollet]

Thanks for the fix! LGTM