Make scikit-learn optional again (#596)

Author: hcho3

python/treelite/sklearn/exporter.py

@@ -4,8 +4,6 @@
 from typing import Any
 
 import numpy as np
-from sklearn.ensemble import RandomForestClassifier
-from sklearn.tree import DecisionTreeClassifier, DecisionTreeRegressor
 
 from ..core import TreeliteError
 from ..model import Model
@@ -61,6 +59,7 @@ def _export_tree(
     # pylint: disable=too-many-locals
     try:
         from sklearn import __version__ as sklearn_version
+        from sklearn.tree import DecisionTreeClassifier
         from sklearn.tree._tree import Tree as SKLearnTree
     except ImportError as e:
         raise TreeliteError("This function requires scikit-learn package") from e
@@ -126,7 +125,7 @@ def _export_tree(
     return subestimator
 
 
-def export_model(model: Model):
+def export_model(model: Model) -> Any:
     """
     Export a model as a scikit-learn RandomForest.
 
@@ -153,7 +152,8 @@ def export_model(model: Model):
     # pylint: disable=too-many-locals
     try:
         from sklearn import __version__ as sklearn_version
-        from sklearn.ensemble import RandomForestRegressor
+        from sklearn.ensemble import RandomForestClassifier, RandomForestRegressor
+        from sklearn.tree import DecisionTreeClassifier, DecisionTreeRegressor
     except ImportError as e:
         raise TreeliteError("This function requires scikit-learn package") from e