`ak.min/max/argmin/argmax` do not work on `axis=1` for the `jax` backend if the last element of an input `ListOffsetArray` is empty

[ikrommyd]

Example:

import awkward as ak
ak.jax.register_and_check()

ak.min(ak.Array([[1, 2], []], backend="cpu"), axis=1) #works
ak.min(ak.Array([[1, 2], []], backend="jax"), axis=1) #errors

All ak.min/max/argmin/argmax are going to give you

File ~/Dropbox/work/coffea_dev/awkward/src/awkward/contents/listoffsetarray.py:1618, in ListOffsetArray._reduce_next(self, reducer, negaxis, starts, shifts, parents, outlength, mask, keepdims, behavior)
   1615 trimmed = self._content[self.offsets[0] : self.offsets[-1]]
   1616 nextstarts = self.offsets[:-1]
-> 1618 outcontent = trimmed._reduce_next(
   1619     reducer,
   1620     negaxis,
   1621     nextstarts,
   1622     shifts,
   1623     nextparents,
   1624     globalstarts_length,
   1625     mask,
   1626     keepdims,
   1627     behavior,
   1628 )
   1630 outoffsets = Index64.empty(outlength + 1, nplike)
   1631 assert outoffsets.nplike is nplike and parents.nplike is nplike

File ~/Dropbox/work/coffea_dev/awkward/src/awkward/contents/numpyarray.py:1155, in NumpyArray._reduce_next(self, reducer, negaxis, starts, shifts, parents, outlength, mask, keepdims, behavior)
   1139     assert (
   1140         outmask.nplike is self._backend.nplike
   1141         and parents.nplike is self._backend.nplike
   1142     )
   1143     self._backend.maybe_kernel_error(
   1144         self._backend[
   1145             "awkward_NumpyArray_reduce_mask_ByteMaskedArray_64",
   (...)   1153         )
   1154     )
-> 1155     out = ak.contents.ByteMaskedArray(outmask, out, False, parameters=None)
   1157 if keepdims:
   1158     out = ak.contents.RegularArray(out, 1, self.length, parameters=None)

File ~/Dropbox/work/coffea_dev/awkward/src/awkward/contents/bytemaskedarray.py:140, in ByteMaskedArray.__init__(self, mask, content, valid_when, parameters)
    131     raise TypeError(
    132         f"{type(self).__name__} 'valid_when' must be boolean, not {valid_when!r}"
    133     )
    134 if (
    135     content.backend.nplike.known_data
    136     and mask.length is not unknown_length
    137     and content.length is not unknown_length
    138     and mask.length > content.length
    139 ):
--> 140     raise ValueError(
    141         f"{type(self).__name__} len(mask) ({mask.length}) must be <= len(content) ({content.length})"
    142     )
    144 assert mask.nplike is content.backend.nplike
    146 self._mask = mask

ValueError: ByteMaskedArray len(mask) (2) must be <= len(content) (1)

This error occurred while calling

    ak.min(
        <Array [[1, 2], []] type='2 * var * int64'>
        axis = 1
    )

caused through the reduce_next call.