LightGBM

[R-package] Add explicit return statement to R functions. (#3703)

* add explicit return statement to functions in callback.R

* add explicit return statement to functions in lgb.Booster.R

* add explicit return statement to functions in lgb.Dataset.R

* add explicit return statement to functions in lgb.Predictor.R

* add explicit return statement to functions in lgb.cv.R

* add explicit return statement to functions in lgb.interprete.R

* add explicit return statement to functions in lgb.plot.importance.R

* add explicit return statement to functions in saveRDS.lgb.Booster.R

* add explicit return statement to functions in utils.R

* add explicit return statement to functions in lgb.plot.interpretation.R

* add explicit return statement to functions in build_r.R

* fix typo

* return(self) -> return(invisible(self))

* fix some inconsistent indention

* fix test failure

* add another return() statement

* fix linting errors