[MINOR][DOCS] Add required imports to CV, train validation split Pyspark ML examples

[srowen]

What changes were proposed in this pull request?

Add required imports to Pyspark ML examples in CrossValidator, TrainValidationSplit

Why are the changes needed?

The examples pass doctests because of previous imports, but as they appear in Pyspark documentation, are incomplete. The additional imports are required to make the example work.

Does this PR introduce any user-facing change?

No, docs only change.

How was this patch tested?

Existing tests.

[SparkQA]

Test build #138567 has finished for PR 32554 at commit 62a333a.

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

[SparkQA]

Test build #138568 has finished for PR 32554 at commit b002cea.

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

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43088/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43088/

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43089/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43089/

[SparkQA]

Test build #138569 has finished for PR 32554 at commit eddb4b7.

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

[SparkQA]

Kubernetes integration test unable to build dist.

exiting with code: 1
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43090/

[srowen]

Merged to master