## What changes were proposed in this pull request?
While trying to create a PR (which was not an issue at the end), I just corrected some style nits.
So, I removed the changes except for some coding style corrections.
- According to the [scala-style-guide#documentation-style](https://github.com/databricks/scala-style-guide#documentation-style), Scala style comments are discouraged.
>```scala
>/** This is a correct one-liner, short description. */
>
>/**
> * This is correct multi-line JavaDoc comment. And
> * this is my second line, and if I keep typing, this would be
> * my third line.
> */
>
>/** In Spark, we don't use the ScalaDoc style so this
> * is not correct.
> */
>```
- Double newlines between consecutive methods was removed. According to [scala-style-guide#blank-lines-vertical-whitespace](https://github.com/databricks/scala-style-guide#blank-lines-vertical-whitespace), single newline appears when
>Between consecutive members (or initializers) of a class: fields, constructors, methods, nested classes, static initializers, instance initializers.
- Remove uesless parentheses in tests
- Use `mapPartitions` instead of `mapPartitionsWithIndex()`.
## How was this patch tested?
Unit tests were used and `dev/run_tests` for style tests.
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#12109 from HyukjinKwon/SPARK-14271.
d7982a3a9a