[SPARK-25321][ML] Revert SPARK-14681 to avoid API breaking change

[WeichenXu123]

What changes were proposed in this pull request?

Revert SPARK-14681 to avoid API breaking change. PR [SPARK-14681] will break mleap.

How was this patch tested?

N/A

[cloud-fan]

Since this changes public API, shall we also revert it from master?

[SparkQA]

Test build #96352 has finished for PR 22492 at commit d976a7e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• sealed abstract class Node extends Serializable

[WeichenXu123]

@mengxr Should this be put into master ?

[mengxr]

We can keep it in master if the next release is 3.0.

[mengxr]

LGTM. Merged into branch-2.4. @WeichenXu123 Next time please create dedicated JIRAs for each QA task PR. Thanks!

[mengxr]

@WeichenXu123 Please close this PR manually. Thanks!

[cloud-fan]

The next version is very likely to be 2.5.0...

[mengxr]

@WeichenXu123 @cloud-fan I made #22618 to revert the change in master.

[dongjoon-hyun]

May I ask a question, @mengxr ? The master becomes 3.0 today by @gatorsmile 's PR and I saw the following your comment here.

We can keep it in master if the next release is 3.0.

Is there any new reason to change the decision?

[mengxr]

@cloud-fan said above the next version is very likely to be 2.5.0 instead of 3.0. Well the next version number is not fully discussed yet. For that reason, I think we should revert the changes in master as well. After that we should check if the feature itself can be added without introducing breaking changes.

[dongjoon-hyun]

Thank you! Although master becomes 3.0.0 already and seems to start to target Mar 2019, I agree with you on reverting for rechecking, @mengxr . Spark 3.0.0 has better have the minimal number of incompatible APIs.