[SPARK-18274][ML][PYSPARK] Memory leak in PySpark JavaWrapper

## What changes were proposed in this pull request?
In`JavaWrapper `'s destructor make Java Gateway dereference object in destructor, using `SparkContext._active_spark_context._gateway.detach`
Fixing the copying parameter bug, by moving the `copy` method from `JavaModel` to `JavaParams`

## How was this patch tested?
```scala
import random, string
from pyspark.ml.feature import StringIndexer

l = [(''.join(random.choice(string.ascii_uppercase) for _ in range(10)), ) for _ in range(int(7e5))]  # 700000 random strings of 10 characters
df = spark.createDataFrame(l, ['string'])

for i in range(50):
    indexer = StringIndexer(inputCol='string', outputCol='index')
    indexer.fit(df)
```
* Before: would keep StringIndexer strong reference, causing GC issues and is halted midway
After: garbage collection works as the object is dereferenced, and computation completes
* Mem footprint tested using profiler
* Added a parameter copy related test which was failing before.

Author: Sandeep Singh <sandeep@techaddict.me>
Author: jkbradley <joseph.kurata.bradley@gmail.com>

Closes #15843 from techaddict/SPARK-18274.

python/pyspark/ml/tests.py

@@ -390,6 +390,24 @@ class ParamTests(PySparkTestCase):
         self.assertEqual(model.getWindowSize(), 6)
 
 
+class EvaluatorTests(SparkSessionTestCase):
+
+    def test_java_params(self):
+        """
+        This tests a bug fixed by SPARK-18274 which causes multiple copies
+        of a Params instance in Python to be linked to the same Java instance.
+        """
+        evaluator = RegressionEvaluator(metricName="r2")
+        df = self.spark.createDataFrame([Row(label=1.0, prediction=1.1)])
+        evaluator.evaluate(df)
+        self.assertEqual(evaluator._java_obj.getMetricName(), "r2")
+        evaluatorCopy = evaluator.copy({evaluator.metricName: "mae"})
+        evaluator.evaluate(df)
+        evaluatorCopy.evaluate(df)
+        self.assertEqual(evaluator._java_obj.getMetricName(), "r2")
+        self.assertEqual(evaluatorCopy._java_obj.getMetricName(), "mae")
+
+
 class FeatureTests(SparkSessionTestCase):
 
     def test_binarizer(self):

python/pyspark/ml/wrapper.py

@@ -71,6 +71,10 @@ class JavaParams(JavaWrapper, Params):
 
     __metaclass__ = ABCMeta
 
+    def __del__(self):
+        if SparkContext._active_spark_context:
+            SparkContext._active_spark_context._gateway.detach(self._java_obj)
+
     def _make_java_param_pair(self, param, value):
         """
         Makes a Java parm pair.
@@ -180,6 +184,25 @@ class JavaParams(JavaWrapper, Params):
                                       % stage_name)
         return py_stage
 
+    def copy(self, extra=None):
+        """
+        Creates a copy of this instance with the same uid and some
+        extra params. This implementation first calls Params.copy and
+        then make a copy of the companion Java pipeline component with
+        extra params. So both the Python wrapper and the Java pipeline
+        component get copied.
+
+        :param extra: Extra parameters to copy to the new instance
+        :return: Copy of this instance
+        """
+        if extra is None:
+            extra = dict()
+        that = super(JavaParams, self).copy(extra)
+        if self._java_obj is not None:
+            that._java_obj = self._java_obj.copy(self._empty_java_param_map())
+            that._transfer_params_to_java()
+        return that
+
 
 @inherit_doc
 class JavaEstimator(JavaParams, Estimator):
@@ -256,21 +279,3 @@ class JavaModel(JavaTransformer, Model):
         super(JavaModel, self).__init__(java_model)
         if java_model is not None:
             self._resetUid(java_model.uid())
-
-    def copy(self, extra=None):
-        """
-        Creates a copy of this instance with the same uid and some
-        extra params. This implementation first calls Params.copy and
-        then make a copy of the companion Java model with extra params.
-        So both the Python wrapper and the Java model get copied.
-
-        :param extra: Extra parameters to copy to the new instance
-        :return: Copy of this instance
-        """
-        if extra is None:
-            extra = dict()
-        that = super(JavaModel, self).copy(extra)
-        if self._java_obj is not None:
-            that._java_obj = self._java_obj.copy(self._empty_java_param_map())
-            that._transfer_params_to_java()
-        return that