[Ml] SPARK-5796 Don't transform data on a last estimator in Pipeline

If it's a last estimator in Pipeline there's no need to transform data, since there's no next stage that would consume this data.

Author: Peter Rudenko <petro.rudenko@gmail.com>

Closes #4590 from petro-rudenko/patch-1 and squashes the following commits:

d13ec33 [Peter Rudenko] [Ml] SPARK-5796 Don't transform data on a last estimator in Pipeline