[SPARK-15788][PYSPARK][ML] PySpark IDFModel missing "idf" property

## What changes were proposed in this pull request?

add method idf to IDF in pyspark

## How was this patch tested?

add unit test

Author: Jeff Zhang <[email protected]>

Closes #13540 from zjffdu/SPARK-15788.

(cherry picked from commit e594b49)
Signed-off-by: Nick Pentreath <[email protected]>

Author: zjffdu

python/pyspark/ml/feature.py

@@ -585,6 +585,8 @@ class IDF(JavaEstimator, HasInputCol, HasOutputCol, JavaMLReadable, JavaMLWritab
     ...     (DenseVector([0.0, 1.0]),), (DenseVector([3.0, 0.2]),)], ["tf"])
     >>> idf = IDF(minDocFreq=3, inputCol="tf", outputCol="idf")
     >>> model = idf.fit(df)
+    >>> model.idf
+    DenseVector([0.0, 0.0])
     >>> model.transform(df).head().idf
     DenseVector([0.0, 0.0])
     >>> idf.setParams(outputCol="freqs").fit(df).transform(df).collect()[1].freqs
@@ -658,6 +660,14 @@ class IDFModel(JavaModel, JavaMLReadable, JavaMLWritable):
     .. versionadded:: 1.4.0
     """
 
+    @property
+    @since("2.0.0")
+    def idf(self):
+        """
+        Returns the IDF vector.
+        """
+        return self._call_java("idf")
+
 
 @inherit_doc
 class MaxAbsScaler(JavaEstimator, HasInputCol, HasOutputCol, JavaMLReadable, JavaMLWritable):