## What changes were proposed in this pull request?

This change pulls the `LogisticAggregator` class out of LogisticRegression.scala and makes it extend `DifferentiableLossAggregator`. It also changes logistic regression to use the generic `RDDLossFunction` instead of having its own.

Other minor changes:
* L2Regularization accepts `Option[Int => Double]` for features standard deviation
* L2Regularization uses `Vector` type instead of Array
* Some tests added to LeastSquaresAggregator

## How was this patch tested?

Unit test suites are added.

Author: sethah <shendrickson@cloudera.com>

Closes #18305 from sethah/SPARK-20988.