refactor: migrate topk finish_example

[jackgerrits]

No description provided.

[jackgerrits]

This one requires more thought:

• What is the correct prediction type here?
• What is the correct way to update shared data and do the printout here?

[jackgerrits]

This one requires more thought:

• What is the correct prediction type here?
• What is the correct way to update shared data and do the printout here?

In general, the changes here maintain semantics while fixing some of the contract breaks.

• The reduction used to pass back predictions as scalars on each example, this has been converted to scalars on the 0th example.
• Updating shared data is the same as before, print update does a single print update per predict vs one per example as was done before

[zwd-ms]

Type change looks good to me