fix: best_model_for_estimator returns inconsistent feature_importances_ compared to automl.model

[murunlin]

Why are these changes needed?

These changes address the issue with inconsistent feature_importances_, proposed by issue #1422
In previous versions:
the best model for each estimator stored in _search_states, the function best_model_for_estimator() returns model in _search_states without checking whether it matches the final best model, so it may return intermediate result.

This modification:
when the estimator matches the automl.best_estimator, directly return automl.model, which ensure the consistency between two feature_importances_ attributes.

Related issue number

Related to #1422

Checks

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• I've added tests (if relevant) corresponding to the changes introduced in this PR.
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[thinkall]

Thank you, @murunlin, for the PR! LGTM. There are some test failures, please help fix them.

[thinkall]

Thanks. LGTM.