Add type_check for quantize

[james77777778]

Related to keras-team/keras-hub#1670
The logic could be further simplified if we could call super().quantize in subclasses.

A minor simplification in the tests for quantization is also included.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.08%. Comparing base (d910dcb) to head (27740b6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #19949   +/-   ##
=======================================
  Coverage   79.07%   79.08%           
=======================================
  Files         499      499           
  Lines       46726    46732    +6     
  Branches     8617     8620    +3     
=======================================
+ Hits        36950    36956    +6     
  Misses       8043     8043           
  Partials     1733     1733           

Flag	Coverage Δ
keras	78.93% <100.00%> (+<0.01%)	⬆️
keras-jax	62.27% <100.00%> (+<0.01%)	⬆️
keras-numpy	57.29% <91.66%> (+<0.01%)	⬆️
keras-tensorflow	63.56% <100.00%> (+<0.01%)	⬆️
keras-torch	62.33% <100.00%> (+<0.01%)	⬆️

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[fchollet]

LGTM, thank you for the PR.