[SPARK-29565][ML][PYTHON] OneHotEncoder should support single-column input/output

[huaxingao]

What changes were proposed in this pull request?

add single-column input/ouput support in OneHotEncoder

Why are the changes needed?

Currently, OneHotEncoder only has multi columns support. It makes sense to support single column as well.

Does this PR introduce any user-facing change?

Yes
OneHotEncoder.setInputCol
OneHotEncoder.setOutputCol

How was this patch tested?

Unit test

[SparkQA]

Test build #112695 has finished for PR 26265 at commit 685b7e1.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• class _OneHotEncoderParams(HasInputCol, HasInputCols, HasOutputCol, HasOutputCols,

[srowen]

It seems OK for consistency. I guess however you can already specify just one input column? and is it more usual to one-hot encode probably all categoricals columns at once? I don't mind either way: making it consistent, or arguing this is a bit different and leaving it.

[huaxingao]

@srowen Thanks for your comment. I prefer to add the single column support for the consistency and completeness of the APIs. Also, in the use cases of one-hot encoding a single column, it's simpler and more convenient to use an API of one input column.

[zhengruifeng]

@huaxingao You need to rebase this PR.

[SparkQA]

Test build #112820 has finished for PR 26265 at commit f3fa47d.

• This patch fails Python style tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• class _OneHotEncoderParams(HasInputCol, HasInputCols, HasOutputCol, HasOutputCols,

[SparkQA]

Test build #112821 has finished for PR 26265 at commit 9d5d6ff.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[viirya]

Thanks! Merging to master.

[huaxingao]

Thanks! @viirya @srowen @zhengruifeng