[SPARK-20764][ML][PYSPARK] Fix visibility discrepancy with numInstances and degreesOfFreedom in LR and GLR - Python version

## What changes were proposed in this pull request?

SPARK-20097 exposed degreesOfFreedom in LinearRegressionSummary and numInstances in GeneralizedLinearRegressionSummary. Python API should be updated to reflect these changes.

## How was this patch tested?
The existing UT

Author: Peng <[email protected]>

Closes #18062 from mpjlu/spark-20764.

Author: Peng

python/pyspark/ml/regression.py

@@ -323,6 +323,14 @@ def numInstances(self):
         """
         return self._call_java("numInstances")
 
+    @property
+    @since("2.2.0")
+    def degreesOfFreedom(self):
+        """
+        Degrees of freedom.
+        """
+        return self._call_java("degreesOfFreedom")
+
     @property
     @since("2.0.0")
     def devianceResiduals(self):
@@ -1565,6 +1573,14 @@ def predictionCol(self):
         """
         return self._call_java("predictionCol")
 
+    @property
+    @since("2.2.0")
+    def numInstances(self):
+        """
+        Number of instances in DataFrame predictions.
+        """
+        return self._call_java("numInstances")
+
     @property
     @since("2.0.0")
     def rank(self):