fix: don't project categorical in ak._v2.packed

[agoose77]

Fixes #1688 by only packing the Indexed[Option]Array's contents if it is a categorical type.

In the long run, it would be nice if we could mark a layout node as non-transient so that we can implement support for this without hard-coding a test for __array__ == "categorical"

[jpivarski]

Great! For the record (we talked about this on Slack), we decided to keep this simple and not remove unused elements from the content. Also, @ivirshup, who raised the issue, argued against it in #1688 (comment).

[jpivarski]

Also for the sake of history, this was @agoose77's attempt to remove unused contents:

        if self.parameter("__array__") == "categorical":
            index_nplike = self._nplike.index_nplike
            index = index_nplike.asarray(self._index.data)
            mask = index_nplike.zeros(len(self._content), dtype=np.bool_)
            mask[index] = True
            tag_valid, = index_nplike.nonzero(mask)
            index_valid = index_nplike.searchsorted(tag_valid, index)
            return IndexedArray(
                ak._v2.index.Index(index_valid),
                self._content._carry(ak._v2.index.Index(tag_valid), False),
                self._identifier,
                self._parameters,
                self._nplike
            )

and I was thinking about it along these lines:

>>> index = np.array([4, 2, 6, 1, 5, 5, 2])
>>> content = np.array([0.0, 1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7, 8.8, 9.9])
>>> uniques, inverse = np.unique(index, return_inverse=True)
>>> uniques
array([1, 2, 4, 5, 6])
>>> inverse
array([2, 1, 4, 0, 3, 3, 1])
>>> uniques[inverse]
array([4, 2, 6, 1, 5, 5, 2])

Both of these are O(n log n), either in the np.searchsorted or in the np.unique (because it sorts). For what we want, it wouldn't have to sort, but we might have needed to write a custom kernel to actually get O(n).

[jpivarski]

Since this merged PR is based on a branch named agoose77/fix-categorical-packed-2, would it be safe to say that agoose77/fix-categorical-packed can be deleted? If so, please do it. (I'm cleaning up dead branches.)

[agoose77]

Done!