[data] drop columns before grouping by in Dataset.unique()

[Zandew]

Why are these changes needed?

Drops non-group-by columns before aggregation for Dataset.unique so that we do not have to shuffle with extra columns that will get dropped later anyways.

Related issue number

First part of #38764

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[scottjlee]

LGTM, can we update the title to include unique(), and update the description to be more general? (e.g. the dataset will not always have images, but rather we are dropping potentially large columns to improve performance)