Remove direct Scikit-learn imports (#4431)

* FIX Remove hard sklearn imports

* FIX Missing whitespace

* FIX minor error

* FIX PEP8 fixes

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

@@ -15,8 +15,11 @@
 import functools
 import timeit
 import numbers
-from sklearn.base import clone
-from sklearn.utils import Bunch
+from cuml.common.import_utils import has_sklearn
+
+if has_sklearn():
+    from sklearn.base import clone
+    from sklearn.utils import Bunch
 from contextlib import contextmanager
 from collections import defaultdict
 import warnings
@@ -30,6 +33,7 @@
 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 +553,9 @@ 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

@@ -15,14 +15,16 @@
 #
 
 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__ = """
-This code is developed and maintained by scikit-learn and imported
-by cuML to maintain the familiar sklearn namespace structure.
-cuML includes tests to ensure full compatibility of these wrappers
-with CUDA-based data and cuML estimators, but all of the underlying code
-is due to the scikit-learn developers.\n\n""" + GridSearchCV.__doc__
+    GridSearchCV.__doc__ = """
+    This code is developed and maintained by scikit-learn and imported
+    by cuML to maintain the familiar sklearn namespace structure.
+    cuML includes tests to ensure full compatibility of these wrappers
+    with CUDA-based data and cuML estimators, but all of the underlying code
+    is due to the scikit-learn developers.\n\n""" + GridSearchCV.__doc__
 
 __all__ = ['train_test_split', 'GridSearchCV']

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