[SPARK-6940] [MLLIB] Add CrossValidator to Python ML pipeline API

Since CrossValidator is a meta algorithm, we copy the implementation in Python. jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #5926 from mengxr/SPARK-6940 and squashes the following commits:

6af181f [Xiangrui Meng] add TODOs
8285134 [Xiangrui Meng] update doc
060f7c3 [Xiangrui Meng] update doctest
acac727 [Xiangrui Meng] add keyword args
cdddecd [Xiangrui Meng] add CrossValidator in Python

1 files changed, 11 insertions, 2 deletions

diff --git a/python/pyspark/ml/pipeline.py b/python/pyspark/ml/pipeline.py
index 7b875e4b71..c1b2077c98 100644
--- a/python/pyspark/ml/pipeline.py
+++ b/python/pyspark/ml/pipeline.py
@@ -22,7 +22,7 @@ from pyspark.ml.util import keyword_only
 from pyspark.mllib.common import inherit_doc
 
 
-__all__ = ['Estimator', 'Transformer', 'Pipeline', 'PipelineModel', 'Evaluator']
+__all__ = ['Estimator', 'Transformer', 'Pipeline', 'PipelineModel', 'Evaluator', 'Model']
 
 
 @inherit_doc
@@ -71,6 +71,15 @@ class Transformer(Params):
 
 
 @inherit_doc
+class Model(Transformer):
+    """
+    Abstract class for models that are fitted by estimators.
+    """
+
+    __metaclass__ = ABCMeta
+
+
+@inherit_doc
 class Pipeline(Estimator):
     """
     A simple pipeline, which acts as an estimator. A Pipeline consists
@@ -154,7 +163,7 @@ class Pipeline(Estimator):
 
 
 @inherit_doc
-class PipelineModel(Transformer):
+class PipelineModel(Model):
     """
     Represents a compiled pipeline with transformers and fitted models.
     """