Remove direct Scikit-learn imports

python/cuml/_thirdparty/sklearn/preprocessing/_column_transformer.py

@@ -30,6 +30,7 @@
 import cuml
 from cuml.internals.global_settings import _global_settings_data
 from cuml.common.array_sparse import SparseCumlArray
+from cuml.common.import_utils import has_sklearn
 from cuml.internals import _deprecate_pos_args
 from ..utils.skl_dependencies import TransformerMixin, BaseComposition, \
     BaseEstimator
@@ -549,6 +550,9 @@ def __init__(self,
                  n_jobs=None,
                  transformer_weights=None,
                  verbose=False):
+        if not has_sklearn():
+            raise ImportError("Scikit-learn is needed to use the "
+                              "Column Transformer")
         if not transformers:
             warnings.warn('Transformers are required')
         self.transformers = transformers

python/cuml/model_selection/__init__.py

@@ -15,7 +15,10 @@
 #
 
 from cuml.model_selection._split import train_test_split
-from sklearn.model_selection import GridSearchCV
+from cuml.common.import_utils import has_sklearn
+
+if has_sklearn():
+    from sklearn.model_selection import GridSearchCV
 
 
 GridSearchCV.__doc__ = """

python/cuml/multiclass/multiclass.py

@@ -14,10 +14,10 @@
 #
 
 import cuml.internals
-import sklearn.multiclass
 
 from cuml.common.array import CumlArray
 from cuml.common.base import Base
+from cuml.common.import_utils import has_sklearn
 from cuml.common.mixins import ClassifierMixin
 from cuml.common.doc_utils import generate_docstring
 from cuml.common import input_to_host_array
@@ -112,6 +112,10 @@ def fit(self, X, y) -> 'MulticlassClassifier':
         """
         Fit a multiclass classifier.
         """
+        if not has_sklearn():
+            raise ImportError("Scikit-learn is needed to use "
+                              "MulticlassClassifier derived classes.")
+        import sklearn.multiclass
         if self.strategy == 'ovr':
             self.multiclass_estimator = sklearn.multiclass.\
                 OneVsRestClassifier(self.estimator, n_jobs=None)

python/cuml/pipeline/__init__.py

@@ -14,7 +14,10 @@
 # limitations under the License.
 #
 
-from sklearn.pipeline import Pipeline, make_pipeline
+from cuml.common.import_utils import has_sklearn
+
+if has_sklearn():
+    from sklearn.pipeline import Pipeline, make_pipeline
 
 disclaimer = """
 This code is developed and maintained by scikit-learn and imported