[SPARK-8051] [MLLIB] make StringIndexerModel silent if input column does not exist

[mengxr]

This is just a workaround to a bigger problem. Some pipeline stages may not be effective during prediction, and they should not complain about missing required columns, e.g. StringIndexerModel. @jkbradley

[SparkQA]

Test build #34023 has finished for PR 6595 at commit e112394.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[jkbradley]

As long as this is aimed at 1.4.1 (not 1.4.0), should we design a better fix? My suggestion would be to have PipelineStage include a Param specifying whether to use it during transform(). Stages could be used in transform() by default, but certain Transformers could override the default to skip during transform(). PipelineModel could read the Param and handle each stage accordingly.

If that's too big a change for 1.4.1, then this temp fix seems tolerable.

Note: We should document the behavior in the docs.

[SparkQA]

Test build #34025 has finished for PR 6595 at commit 8ee7c7e.

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

[mengxr]

I checked the transformers we have. Perhaps this is the only one that would operate on target labels, and it is blocking users from making predictions without labels. So it would be nice to merge this fix into branch-1.4, before 1.4.1 is out.

[SparkQA]

Test build #34069 has finished for PR 6595 at commit b6a36b9.

• This patch fails MiMa tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class ElementwiseProduct(val scalingVec: Vector) extends VectorTransformer

[jkbradley]

LGTM pending tests

[jkbradley]

test this please

[SparkQA]

Test build #34106 has finished for PR 6595 at commit b6a36b9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class ElementwiseProduct(val scalingVec: Vector) extends VectorTransformer

[jkbradley]

Merging with master and branch-1.4