Returns the feature / variable / column names from a fitted
booster object, which are set automatically during the call to xgb.train()
from the DMatrix names, or which can be set manually through setinfo().
If the object doesn't have feature names, will return NULL.
It is equivalent to calling getinfo(object, "feature_name").
Usage
# S3 method for class 'xgb.Booster'
variable.names(object, ...)