[SPARK-18667][PYSPARK][SQL] Change the way to group row in BatchEvalPythonExec...

[SPARK-18667][PYSPARK][SQL] Change the way to group row in BatchEvalPythonExec so input_file_name function can work with UDF in pyspark

## What changes were proposed in this pull request?

`input_file_name` doesn't return filename when working with UDF in PySpark. An example shows the problem:

    from pyspark.sql.functions import *
    from pyspark.sql.types import *

    def filename(path):
        return path

    sourceFile = udf(filename, StringType())
    spark.read.json("tmp.json").select(sourceFile(input_file_name())).show()

    +---------------------------+
    |filename(input_file_name())|
    +---------------------------+
    |                           |
    +---------------------------+

The cause of this issue is, we group rows in `BatchEvalPythonExec` for batching processing of PythonUDF. Currently we group rows first and then evaluate expressions on the rows. If the data is less than the required number of rows for a group, the iterator will be consumed to the end before the evaluation. However, once the iterator reaches the end, we will unset input filename. So the input_file_name expression can't return correct filename.

This patch fixes the approach to group the batch of rows. We evaluate the expression first and then group evaluated results to batch.

## How was this patch tested?

Added unit test to PySpark.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #16115 from viirya/fix-py-udf-input-filename.