Add a warning for mismatched inputs structure in Functional

[james77777778]

There are several issues related to mismatched inputs structure:

• This works in keras 2.15 with TensorFlow 2.15 but does not 3.5 with TensorFlow 2.17: multiple lookup layer types in inputs #20166
• Keras 3 doesn't map dictionary inputs by "key"  #20136
• A possible bug with tf 2.17.0 #20086
  and more...

As a result, it would be beneficial to add a warning for users.
Ultimately, we might want to raise an error when a mismatch occurs. Otherwise, it could lead to subtle issues if the inputs have the same shape and dtype, as the computation could be incorrect even though the code runs.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.34%. Comparing base (3cc4d44) to head (dddfbfe).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20170   +/-   ##
=======================================
  Coverage   79.34%   79.34%           
=======================================
  Files         501      501           
  Lines       47319    47325    +6     
  Branches     8692     8694    +2     
=======================================
+ Hits        37544    37550    +6     
  Misses       8017     8017           
  Partials     1758     1758           

Flag	Coverage Δ
keras	79.19% <100.00%> (+<0.01%)	⬆️
keras-jax	62.47% <100.00%> (+<0.01%)	⬆️
keras-numpy	57.65% <100.00%> (+0.07%)	⬆️
keras-tensorflow	63.85% <100.00%> (+<0.01%)	⬆️
keras-torch	62.50% <100.00%> (+<0.01%)	⬆️

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]

Thanks for the improvement -- LGTM