[SPARK-16831][PYTHON] Fixed bug in CrossValidator.avgMetrics

## What changes were proposed in this pull request?

avgMetrics was summed, not averaged, across folds

Author: =^_^= <maxmoroz@gmail.com>

Closes #14456 from pkch/pkch-patch-1.

python/pyspark/ml/tuning.py

@@ -166,6 +166,8 @@ class CrossValidator(Estimator, ValidatorParams):
     >>> evaluator = BinaryClassificationEvaluator()
     >>> cv = CrossValidator(estimator=lr, estimatorParamMaps=grid, evaluator=evaluator)
     >>> cvModel = cv.fit(dataset)
+    >>> cvModel.avgMetrics[0]
+    0.5
     >>> evaluator.evaluate(cvModel.transform(dataset))
     0.8333...
 
@@ -234,7 +236,7 @@ def _fit(self, dataset):
                 model = est.fit(train, epm[j])
                 # TODO: duplicate evaluator to take extra params from input
                 metric = eva.evaluate(model.transform(validation, epm[j]))
-                metrics[j] += metric
+                metrics[j] += metric/nFolds
 
         if eva.isLargerBetter():
             bestIndex = np.argmax(metrics)