[SPARK-10759][ML] update cross validator with include_example

[JeremyNixon]

This pull request uses {%include_example%} to add an example for the python cross validator to ml-guide.

[mengxr]

ok to test

[mengxr]

cc @yinxusen

[SparkQA]

Test build #51790 has finished for PR 11240 at commit cad4f71.

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

[yinxusen]

@mengxr LGTM

[JeremyNixon]

@mengxr @yinxusen I made an update to an error in the train-validation-split example, it's currently pointing to the wrong data (was overwritten by https://issues.apache.org/jira/browse/SPARK-13012). Let me know if you'd prefer I create a new JIRA instead.

[yinxusen]

@JeremyNixon There is no need to fix those two examples. Actually we have duplicated example code now - JavaModelSelectionViaTrainValidationSplitExample VS JavaTrainValidationSplitExample, etc. We prepare to remove the duplicated JavaTrainValidationSplitExample later under JIRA https://issues.apache.org/jira/browse/SPARK-13461

[SparkQA]

Test build #51812 has finished for PR 11240 at commit 54d2d08.

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

[JeremyNixon]

@yinxusen Great to hear, this PR should be good to go.

[SparkQA]

Test build #51818 has finished for PR 11240 at commit 69aa6d0.

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

[mengxr]

Merged into master. Thanks!

[JeremyNixon]

@mengxr My pleasure!