[SPARK-13338][ML] Allow setting 'degree' parameter to 1 for PolynomialExpansion

[grzegorz-chilkiewicz]

No description provided.

[yanboliang]

LGTM pending test.
ping @mengxr @jkbradley
Could you add @grzegorz-chilkiewicz to white list?

[mengxr]

ok to test

[SparkQA]

Test build #51467 has finished for PR 11216 at commit c66d5e5.

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

[mengxr]

The implementation should work with degree 1, but we need to add a test to show that. @grzegorz-chilkiewicz Could you update this PR and add a test to show with degree 1 it is a no-op? Just testing the setter is not sufficient.

[grzegorz-chilkiewicz]

OK, I'll prepare that test soon.

I've checked that with even degree 0, PolynomialExpansion works (and returns '0-length vector').
Does it make sense to extend range of legal parameters to gtEq(0) ?
(PolynomialExpansion works even for negative integers - and returns 0-length vector...)

[MLnick]

@mengxr I could see value 1 being used in a cross-validation setup. But do we really want to allow <=0 param values? I don't think it makes a lot of sense to return a zero-length vector for a non-zero length input. There's no real use case for this.

[mengxr]

No, it is not necessary to support 0 degree.

[SparkQA]

Test build #51774 has finished for PR 11216 at commit 83f22a9.

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

[mengxr]

LGTM. Merged into master. Thanks!