[SPARK-14500] [ML] Accept Dataset[_] instead of DataFrame in MLlib APIs

## What changes were proposed in this pull request?

This PR updates MLlib APIs to accept `Dataset[_]` as input where `DataFrame` was the input type. This PR doesn't change the output type. In Java, `Dataset[_]` maps to `Dataset<?>`, which includes `Dataset<Row>`. Some implementations were changed in order to return `DataFrame`. Tests and examples were updated. Note that this is a breaking change for subclasses of Transformer/Estimator.

Lol, we don't have to rename the input argument, which has been `dataset` since Spark 1.2.

TODOs:
- [x] update MiMaExcludes (seems all covered by explicit filters from SPARK-13920)
- [x] Python
- [x] add a new test to accept Dataset[LabeledPoint]
- [x] remove unused imports of Dataset

## How was this patch tested?

Exiting unit tests with some modifications.

cc: rxin jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #12274 from mengxr/SPARK-14500.