Error with lightgbm regressor and importance_measure = gini [BUG]

[tszyan-bain]

Describe the bug

Checking the availability of feature_importance_ when importance_measure="gini" does not work for lightgbm regressor.

To Reproduce

Steps to reproduce the behavior:

1. Initiate a lightgbm regressor and use it as the model for BorutaShap
2. choose importance_measure = "gini"
3. the pre-fitting check will false since before doing fitting, feature_importance_ attribute is not set in the regressor

Expected behavior

It should not raise an error.

Screenshots

Additional context

Workaround:

1. use "shap"
2. pre-fitting: doing a pre-fitting will expose the feature_importance_ attribute in the regressor