yzhao062 / pyod / 23972073553

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Fix CD detector train-test split: reuse fitted models at prediction time (#624)

decision_function() previously re-fitted the regression model on test
data instead of using models trained during fit(). This patch stores
per-feature fitted models and reuses them at prediction time.

Additional fixes:
- Fix broken NaN check: (mse != 0) or (mse != np.nan) was always True;
  replaced with (mse != 0) and not np.isnan(mse)
- Use sklearn.base.clone instead of deepcopy for model copying
- Fix mutable default argument model=LinearRegression() in __init__
- Add check_is_fitted guard for _models attribute
- Update docstrings to reflect sklearn clone contract

Tests added:
- Sentinel-based test proving decision_function does not refit
- sklearn.base.clone compatibility test with non-picklable estimator

Based on the idea from PR #624 by Daniel Kulik (@KulikDM), with
corrections applied during code review.

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