Fix SKLearnClassifier and SKLearnRegressor examples

[timovdk]

The examples in the docstrings for SKLearnClassifier and SKLearnRegressor are currently not working. This PR addresses the problems mentioned in, and closes #21384:

• General Fixes:
  • Import layers from keras.layers
  • Import Model from keras.models
  • Remove unnecessary []
• SKLearnClassifier Fix:
  • Use a 2 class classification problem as an example (instead of the 3 class example)
• SKLearnRegressor Fix:
  • Use linear activation because it is a regression problem that does not fall between [0, 1]

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.67%. Comparing base (764ed95) to head (2baee33).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21385   +/-   ##
=======================================
  Coverage   82.67%   82.67%           
=======================================
  Files         565      565           
  Lines       55064    55064           
  Branches     8569     8569           
=======================================
  Hits        45525    45525           
  Misses       7441     7441           
  Partials     2098     2098           

Flag	Coverage Δ
keras	82.48% <ø> (ø)
keras-jax	63.50% <ø> (ø)
keras-numpy	58.67% <ø> (ø)
keras-openvino	33.45% <ø> (ø)
keras-tensorflow	63.91% <ø> (ø)
keras-torch	63.53% <ø> (ø)

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[fchollet]

LGTM, thanks for the fixes