[hotfix][util] fix partition utility

[sjwiesman]

What is the purpose of the change

Fixes the partition utility method to actually partition collections

Verifying this change

Added a UT. I also checked its usages and the only caller simply uses it as a performance optimization so there aren't any other correctness issues that need to be addressed.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Automated Checks

Last check on commit 5ecc2c3 (Wed Dec 16 15:05:40 UTC 2020)

Warnings:

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CI report:

• 5ecc2c3 Azure: PENDING

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