[SPARK-24231][PYSPARK][ML] Provide Python API for evaluateEachIteration for spark.ml GBTs

[lu-wang-dl]

What changes were proposed in this pull request?

Add evaluateEachIteration for GBTClassification and GBTRegressionModel

How was this patch tested?

doctest

Please review http://spark.apache.org/contributing.html before opening a pull request.

[SparkQA]

Test build #90653 has finished for PR 21335 at commit c25c5a6.

• This patch fails Python style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #90655 has finished for PR 21335 at commit 5765049.

• This patch fails Python style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #90657 has finished for PR 21335 at commit d02797c.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #90658 has finished for PR 21335 at commit 17be21c.

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

[mengxr]

LGTM. Merged into master. Thanks!