Mark layers with no state as built.

[james77777778]

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There is an issue reported by Matt where the QLoRA tutorial failed due to unbuilt layers.

The root cause is that the line self.built = True was removed from __init__ in some layers, particularly in Dropout and Softmax. (This line was originally added in this commit: https://github.com/keras-team/keras/commit/7748a6aebba4bbc77a7866c44c1293c4bb9e0808)

This PR adds an assert_built_after_instantiation in the test case to prevent future breakages.

cc @mattdangerw

Note: there are still many stateless layers not included in this PR, but I want to keep this PR manageable for review. These can be addressed in the future.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.38%. Comparing base (3de251d) to head (bdbe197). Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20175      +/-   ##
==========================================
+ Coverage   79.34%   79.38%   +0.03%     
==========================================
  Files         501      501              
  Lines       47325    47341      +16     
  Branches     8694     8692       -2     
==========================================
+ Hits        37550    37581      +31     
+ Misses       8017     8003      -14     
+ Partials     1758     1757       -1     

Flag	Coverage Δ
keras	79.23% <100.00%> (+0.03%)	:arrow_up:
keras-jax	62.50% <70.00%> (+0.03%)	:arrow_up:
keras-numpy	57.67% <56.66%> (+0.03%)	:arrow_up:
keras-tensorflow	63.89% <70.00%> (+0.03%)	:arrow_up:
keras-torch	62.55% <100.00%> (+0.04%)	:arrow_up:

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