[WIP][SPARK-29566][PYTHON][ML][FOLLOWUP] Add setInputCol and setOutputCol to ImputerModel

[zero323]

What changes were proposed in this pull request?

This PR adds setInputCol and setOutputCol methods to ImputerModel

Why are the changes needed?

Feature parity. These methods are already present in Scala:

spark/mllib/src/main/scala/org/apache/spark/ml/feature/Imputer.scala

Lines 116 to 122 in b389b8c

	/** @group setParam */
	@Since("3.0.0")
	def setInputCol(value: String): this.type = set(inputCol, value)
	/** @group setParam */
	@Since("3.0.0")
	def setOutputCol(value: String): this.type = set(outputCol, value)

Does this PR introduce any user-facing change?

PySpark ImputerModel now supports setters for single column workflow.

How was this patch tested?

Manual tests (should we add Python unit tests, as Imputer is not covered at all?).

[SparkQA]

Test build #116666 has finished for PR 27195 at commit 1adc184.

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

[zero323]

Closing as #26247 (comment)