[SPARK-14374] [ML] [PySpark] PySpark ml GBTClassifier, Regressor support export/import

[yanboliang]

What changes were proposed in this pull request?

PySpark ml GBTClassifier, Regressor support export/import.

How was this patch tested?

Doc test.

cc @jkbradley

[SparkQA]

Test build #55798 has finished for PR 12383 at commit 7fe414a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class GBTClassificationModel(TreeEnsembleModels, JavaMLWritable, JavaMLReadable):
  • class GBTRegressionModel(TreeEnsembleModels, JavaMLWritable, JavaMLReadable):

[jkbradley]

Taking a look now

[jkbradley]

LGTM
I'll rerun tests once more to make sure some recent pyspark changes are reflected in the tests

[SparkQA]

Test build #2789 has finished for PR 12383 at commit 7fe414a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class GBTClassificationModel(TreeEnsembleModels, JavaMLWritable, JavaMLReadable):
  • class GBTRegressionModel(TreeEnsembleModels, JavaMLWritable, JavaMLReadable):

[mengxr]

Merged into master. Thanks!