Ensures that the layer is marked as built when the build is not overriden

[james77777778]

Fix #20871

This PR ensures that the layer is marked as built when the build is not overriden.

Additionally, this PR cleans up the redundant self.built = True.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.25%. Comparing base (b5261a0) to head (1e65b51).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
keras/src/testing/test_case.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20880      +/-   ##
==========================================
- Coverage   82.25%   82.25%   -0.01%     
==========================================
  Files         561      561              
  Lines       52680    52665      -15     
  Branches     8144     8146       +2     
==========================================
- Hits        43334    43318      -16     
- Misses       7342     7343       +1     
  Partials     2004     2004              

Flag	Coverage Δ
keras	82.06% <96.55%> (-0.01%)	⬇️
keras-jax	64.16% <93.10%> (-0.03%)	⬇️
keras-numpy	58.97% <68.96%> (-0.03%)	⬇️
keras-openvino	32.55% <37.93%> (+<0.01%)	⬆️
keras-tensorflow	64.81% <93.10%> (-0.01%)	⬇️
keras-torch	64.23% <86.20%> (-0.03%)	⬇️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[fchollet]

This code snippet seems to be recurring a lot -- could it be added to the base class instead?

[james77777778]

Yes, I have moved the logic to Layer._build_at_init() and applied it everywhere.
Note: We cannot directly apply it at the end of Layer.__init__() because it would break some corner cases in the codebase.

[fchollet]

LGTM, thank you!