Fix pandas groupby -> apply warning

[tomaarsen]

Hello!

Pull Request overview

• Fix pandas groupby -> apply warning

Details

Previously, this warning would trigger:

DeprecationWarning: DataFrameGroupBy.apply operated on the grouping columns. This behavior is deprecated, and in a future version of pandas the grouping columns will be excluded from the operation. Either pass `include_groups=False` to exclude the groupings or explicitly select the grouping columns after groupby to silence this warning.

Now, we use head to sample per group. Because the dataset is already shuffled, this is equivalent to using sample as we did before.

• Tom Aarsen

[HuggingFaceDocBuilderDev]

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