[SPARK-21633][ML][PYTHON] UnaryTransformer in Python

## What changes were proposed in this pull request?

Implemented UnaryTransformer in Python.

## How was this patch tested?

This patch was tested by creating a MockUnaryTransformer class in the unit tests that extends UnaryTransformer and testing that the transform function produced correct output.

Author: Ajay Saini <ajays725@gmail.com>

Closes #18746 from ajaysaini725/AddPythonUnaryTransformer.