[SPARK-23248][PYTHON][EXAMPLES] Relocate module docstrings to the top in PySpark examples

## What changes were proposed in this pull request?

This PR proposes to relocate the docstrings in modules of examples to the top. Seems these are mistakes. So, for example, the below codes

```python
>>> help(aft_survival_regression)
```

shows the module docstrings for examples as below:

**Before**

```
Help on module aft_survival_regression:

NAME
    aft_survival_regression

...

DESCRIPTION
    # Licensed to the Apache Software Foundation (ASF) under one or more
    # contributor license agreements.  See the NOTICE file distributed with
    # this work for additional information regarding copyright ownership.
    # The ASF licenses this file to You under the Apache License, Version 2.0
    # (the "License"); you may not use this file except in compliance with
    # the License.  You may obtain a copy of the License at
    #
    #    http://www.apache.org/licenses/LICENSE-2.0
    #
    # Unless required by applicable law or agreed to in writing, software
    # distributed under the License is distributed on an "AS IS" BASIS,
    # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    # See the License for the specific language governing permissions and
    # limitations under the License.
    #

...

(END)
```

**After**

```
Help on module aft_survival_regression:

NAME
    aft_survival_regression

...

DESCRIPTION
    An example demonstrating aft survival regression.
    Run with:
      bin/spark-submit examples/src/main/python/ml/aft_survival_regression.py

(END)
```

## How was this patch tested?

Manually checked.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #20416 from HyukjinKwon/module-docstring-example.

examples/src/main/python/avro_inputformat.py

@@ -15,13 +15,6 @@
 # limitations under the License.
 #
 
-from __future__ import print_function
-
-import sys
-
-from functools import reduce
-from pyspark.sql import SparkSession
-
 """
 Read data file users.avro in local Spark distro:
 
@@ -50,6 +43,13 @@ $ ./bin/spark-submit --driver-class-path /path/to/example/jar \
 {u'favorite_color': None, u'name': u'Alyssa'}
 {u'favorite_color': u'red', u'name': u'Ben'}
 """
+from __future__ import print_function
+
+import sys
+
+from functools import reduce
+from pyspark.sql import SparkSession
+
 if __name__ == "__main__":
     if len(sys.argv) != 2 and len(sys.argv) != 3:
         print("""

examples/src/main/python/ml/aft_survival_regression.py

@@ -15,6 +15,11 @@
 # limitations under the License.
 #
 
+"""
+An example demonstrating aft survival regression.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/aft_survival_regression.py
+"""
 from __future__ import print_function
 
 # $example on$
@@ -23,12 +28,6 @@ from pyspark.ml.linalg import Vectors
 # $example off$
 from pyspark.sql import SparkSession
 
-"""
-An example demonstrating aft survival regression.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/aft_survival_regression.py
-"""
-
 if __name__ == "__main__":
     spark = SparkSession \
         .builder \

examples/src/main/python/ml/bisecting_k_means_example.py

@@ -15,6 +15,11 @@
 # limitations under the License.
 #
 
+"""
+An example demonstrating bisecting k-means clustering.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/bisecting_k_means_example.py
+"""
 from __future__ import print_function
 
 # $example on$
@@ -22,12 +27,6 @@ from pyspark.ml.clustering import BisectingKMeans
 # $example off$
 from pyspark.sql import SparkSession
 
-"""
-An example demonstrating bisecting k-means clustering.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/bisecting_k_means_example.py
-"""
-
 if __name__ == "__main__":
     spark = SparkSession\
         .builder\

examples/src/main/python/ml/bucketed_random_projection_lsh_example.py

@@ -15,7 +15,11 @@
 # limitations under the License.
 #
 
-
+"""
+An example demonstrating BucketedRandomProjectionLSH.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/bucketed_random_projection_lsh_example.py
+"""
 from __future__ import print_function
 
 # $example on$
@@ -25,12 +29,6 @@ from pyspark.sql.functions import col
 # $example off$
 from pyspark.sql import SparkSession
 
-"""
-An example demonstrating BucketedRandomProjectionLSH.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/bucketed_random_projection_lsh_example.py
-"""
-
 if __name__ == "__main__":
     spark = SparkSession \
         .builder \

examples/src/main/python/ml/chi_square_test_example.py

@@ -15,6 +15,11 @@
 # limitations under the License.
 #
 
+"""
+An example for Chi-square hypothesis testing.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/chi_square_test_example.py
+"""
 from __future__ import print_function
 
 from pyspark.sql import SparkSession
@@ -23,11 +28,6 @@ from pyspark.ml.linalg import Vectors
 from pyspark.ml.stat import ChiSquareTest
 # $example off$
 
-"""
-An example for Chi-square hypothesis testing.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/chi_square_test_example.py
-"""
 if __name__ == "__main__":
     spark = SparkSession \
         .builder \

examples/src/main/python/ml/correlation_example.py

@@ -15,6 +15,11 @@
 # limitations under the License.
 #
 
+"""
+An example for computing correlation matrix.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/correlation_example.py
+"""
 from __future__ import print_function
 
 # $example on$
@@ -23,11 +28,6 @@ from pyspark.ml.stat import Correlation
 # $example off$
 from pyspark.sql import SparkSession
 
-"""
-An example for computing correlation matrix.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/correlation_example.py
-"""
 if __name__ == "__main__":
     spark = SparkSession \
         .builder \

examples/src/main/python/ml/cross_validator.py

@@ -15,6 +15,13 @@
 # limitations under the License.
 #
 
+"""
+A simple example demonstrating model selection using CrossValidator.
+This example also demonstrates how Pipelines are Estimators.
+Run with:
+
+  bin/spark-submit examples/src/main/python/ml/cross_validator.py
+"""
 from __future__ import print_function
 
 # $example on$
@@ -26,14 +33,6 @@ from pyspark.ml.tuning import CrossValidator, ParamGridBuilder
 # $example off$
 from pyspark.sql import SparkSession
 
-"""
-A simple example demonstrating model selection using CrossValidator.
-This example also demonstrates how Pipelines are Estimators.
-Run with:
-
-  bin/spark-submit examples/src/main/python/ml/cross_validator.py
-"""
-
 if __name__ == "__main__":
     spark = SparkSession\
         .builder\

examples/src/main/python/ml/fpgrowth_example.py

@@ -15,16 +15,15 @@
 # limitations under the License.
 #
 
-# $example on$
-from pyspark.ml.fpm import FPGrowth
-# $example off$
-from pyspark.sql import SparkSession
-
 """
 An example demonstrating FPGrowth.
 Run with:
   bin/spark-submit examples/src/main/python/ml/fpgrowth_example.py
 """
+# $example on$
+from pyspark.ml.fpm import FPGrowth
+# $example off$
+from pyspark.sql import SparkSession
 
 if __name__ == "__main__":
     spark = SparkSession\

examples/src/main/python/ml/gaussian_mixture_example.py

@@ -15,6 +15,11 @@
 # limitations under the License.
 #
 
+"""
+A simple example demonstrating Gaussian Mixture Model (GMM).
+Run with:
+  bin/spark-submit examples/src/main/python/ml/gaussian_mixture_example.py
+"""
 from __future__ import print_function
 
 # $example on$
@@ -22,12 +27,6 @@ from pyspark.ml.clustering import GaussianMixture
 # $example off$
 from pyspark.sql import SparkSession
 
-"""
-A simple example demonstrating Gaussian Mixture Model (GMM).
-Run with:
-  bin/spark-submit examples/src/main/python/ml/gaussian_mixture_example.py
-"""
-
 if __name__ == "__main__":
     spark = SparkSession\
         .builder\

examples/src/main/python/ml/generalized_linear_regression_example.py

@@ -15,6 +15,11 @@
 # limitations under the License.
 #
 
+"""
+An example demonstrating generalized linear regression.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/generalized_linear_regression_example.py
+"""
 from __future__ import print_function
 
 from pyspark.sql import SparkSession
@@ -22,12 +27,6 @@ from pyspark.sql import SparkSession
 from pyspark.ml.regression import GeneralizedLinearRegression
 # $example off$
 
-"""
-An example demonstrating generalized linear regression.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/generalized_linear_regression_example.py
-"""
-
 if __name__ == "__main__":
     spark = SparkSession\
         .builder\

examples/src/main/python/ml/imputer_example.py

@@ -15,16 +15,15 @@
 # limitations under the License.
 #
 
-# $example on$
-from pyspark.ml.feature import Imputer
-# $example off$
-from pyspark.sql import SparkSession
-
 """
 An example demonstrating Imputer.
 Run with:
   bin/spark-submit examples/src/main/python/ml/imputer_example.py
 """
+# $example on$
+from pyspark.ml.feature import Imputer
+# $example off$
+from pyspark.sql import SparkSession
 
 if __name__ == "__main__":
     spark = SparkSession\

examples/src/main/python/ml/isotonic_regression_example.py

@@ -17,6 +17,9 @@
 
 """
 Isotonic Regression Example.
+
+Run with:
+  bin/spark-submit examples/src/main/python/ml/isotonic_regression_example.py
 """
 from __future__ import print_function
 
@@ -25,12 +28,6 @@ from pyspark.ml.regression import IsotonicRegression
 # $example off$
 from pyspark.sql import SparkSession
 
-"""
-An example demonstrating isotonic regression.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/isotonic_regression_example.py
-"""
-
 if __name__ == "__main__":
     spark = SparkSession\
         .builder\

examples/src/main/python/ml/kmeans_example.py

@@ -15,6 +15,13 @@
 # limitations under the License.
 #
 
+"""
+An example demonstrating k-means clustering.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/kmeans_example.py
+
+This example requires NumPy (http://www.numpy.org/).
+"""
 from __future__ import print_function
 
 # $example on$
@@ -24,14 +31,6 @@ from pyspark.ml.evaluation import ClusteringEvaluator
 
 from pyspark.sql import SparkSession
 
-"""
-An example demonstrating k-means clustering.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/kmeans_example.py
-
-This example requires NumPy (http://www.numpy.org/).
-"""
-
 if __name__ == "__main__":
     spark = SparkSession\
         .builder\

examples/src/main/python/ml/lda_example.py

@@ -15,7 +15,11 @@
 # limitations under the License.
 #
 
-
+"""
+An example demonstrating LDA.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/lda_example.py
+"""
 from __future__ import print_function
 
 # $example on$
@@ -23,12 +27,6 @@ from pyspark.ml.clustering import LDA
 # $example off$
 from pyspark.sql import SparkSession
 
-"""
-An example demonstrating LDA.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/lda_example.py
-"""
-
 if __name__ == "__main__":
     spark = SparkSession \
         .builder \

examples/src/main/python/ml/logistic_regression_summary_example.py

@@ -15,6 +15,11 @@
 # limitations under the License.
 #
 
+"""
+An example demonstrating Logistic Regression Summary.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/logistic_regression_summary_example.py
+"""
 from __future__ import print_function
 
 # $example on$
@@ -22,12 +27,6 @@ from pyspark.ml.classification import LogisticRegression
 # $example off$
 from pyspark.sql import SparkSession
 
-"""
-An example demonstrating Logistic Regression Summary.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/logistic_regression_summary_example.py
-"""
-
 if __name__ == "__main__":
     spark = SparkSession \
         .builder \

examples/src/main/python/ml/min_hash_lsh_example.py

@@ -15,7 +15,11 @@
 # limitations under the License.
 #
 
-
+"""
+An example demonstrating MinHashLSH.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/min_hash_lsh_example.py
+"""
 from __future__ import print_function
 
 # $example on$
@@ -25,12 +29,6 @@ from pyspark.sql.functions import col
 # $example off$
 from pyspark.sql import SparkSession
 
-"""
-An example demonstrating MinHashLSH.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/min_hash_lsh_example.py
-"""
-
 if __name__ == "__main__":
     spark = SparkSession \
         .builder \

examples/src/main/python/ml/one_vs_rest_example.py

@@ -15,6 +15,12 @@
 # limitations under the License.
 #
 
+"""
+An example of Multiclass to Binary Reduction with One Vs Rest,
+using Logistic Regression as the base classifier.
+Run with:
+  bin/spark-submit examples/src/main/python/ml/one_vs_rest_example.py
+"""
 from __future__ import print_function
 
 # $example on$
@@ -23,13 +29,6 @@ from pyspark.ml.evaluation import MulticlassClassificationEvaluator
 # $example off$
 from pyspark.sql import SparkSession
 
-"""
-An example of Multiclass to Binary Reduction with One Vs Rest,
-using Logistic Regression as the base classifier.
-Run with:
-  bin/spark-submit examples/src/main/python/ml/one_vs_rest_example.py
-"""
-
 if __name__ == "__main__":
     spark = SparkSession \
         .builder \

examples/src/main/python/ml/train_validation_split.py

@@ -15,13 +15,6 @@
 # limitations under the License.
 #
 
-# $example on$
-from pyspark.ml.evaluation import RegressionEvaluator
-from pyspark.ml.regression import LinearRegression
-from pyspark.ml.tuning import ParamGridBuilder, TrainValidationSplit
-# $example off$
-from pyspark.sql import SparkSession
-
 """
 This example demonstrates applying TrainValidationSplit to split data
 and preform model selection.
@@ -29,6 +22,12 @@ Run with:
 
   bin/spark-submit examples/src/main/python/ml/train_validation_split.py
 """
+# $example on$
+from pyspark.ml.evaluation import RegressionEvaluator
+from pyspark.ml.regression import LinearRegression
+from pyspark.ml.tuning import ParamGridBuilder, TrainValidationSplit
+# $example off$
+from pyspark.sql import SparkSession
 
 if __name__ == "__main__":
     spark = SparkSession\

examples/src/main/python/parquet_inputformat.py

@@ -15,12 +15,6 @@
 # limitations under the License.
 #
 
-from __future__ import print_function
-
-import sys
-
-from pyspark.sql import SparkSession
-
 """
 Read data file users.parquet in local Spark distro:
 
@@ -35,6 +29,12 @@ $ ./bin/spark-submit --driver-class-path /path/to/example/jar \\
 {u'favorite_color': u'red', u'name': u'Ben', u'favorite_numbers': []}
 <...more log output...>
 """
+from __future__ import print_function
+
+import sys
+
+from pyspark.sql import SparkSession
+
 if __name__ == "__main__":
     if len(sys.argv) != 2:
         print("""

examples/src/main/python/sql/basic.py

@@ -15,6 +15,11 @@
 # limitations under the License.
 #
 
+"""
+A simple example demonstrating basic Spark SQL features.
+Run with:
+  ./bin/spark-submit examples/src/main/python/sql/basic.py
+"""
 from __future__ import print_function
 
 # $example on:init_session$
@@ -30,12 +35,6 @@ from pyspark.sql import Row
 from pyspark.sql.types import *
 # $example off:programmatic_schema$
 
-"""
-A simple example demonstrating basic Spark SQL features.
-Run with:
-  ./bin/spark-submit examples/src/main/python/sql/basic.py
-"""
-
 
 def basic_df_example(spark):
     # $example on:create_df$

examples/src/main/python/sql/datasource.py

@@ -15,6 +15,11 @@
 # limitations under the License.
 #
 
+"""
+A simple example demonstrating Spark SQL data sources.
+Run with:
+  ./bin/spark-submit examples/src/main/python/sql/datasource.py
+"""
 from __future__ import print_function
 
 from pyspark.sql import SparkSession
@@ -22,12 +27,6 @@ from pyspark.sql import SparkSession
 from pyspark.sql import Row
 # $example off:schema_merging$
 
-"""
-A simple example demonstrating Spark SQL data sources.
-Run with:
-  ./bin/spark-submit examples/src/main/python/sql/datasource.py
-"""
-
 
 def basic_datasource_example(spark):
     # $example on:generic_load_save_functions$

examples/src/main/python/sql/hive.py

@@ -15,6 +15,11 @@
 # limitations under the License.
 #
 
+"""
+A simple example demonstrating Spark SQL Hive integration.
+Run with:
+  ./bin/spark-submit examples/src/main/python/sql/hive.py
+"""
 from __future__ import print_function
 
 # $example on:spark_hive$
@@ -24,12 +29,6 @@ from pyspark.sql import SparkSession
 from pyspark.sql import Row
 # $example off:spark_hive$
 
-"""
-A simple example demonstrating Spark SQL Hive integration.
-Run with:
-  ./bin/spark-submit examples/src/main/python/sql/hive.py
-"""
-
 
 if __name__ == "__main__":
     # $example on:spark_hive$