Fix argsort for views with nonzero data offsets

[vera-rubin]

Checklist

• The title and commit message(s) are descriptive.
• Small commits made to fix your PR have been squashed to avoid history pollution.
• Tests have been added for new features or bug fixes.
• API of new functions and classes are documented. (Not applicable, no new API.)

Description

xt::argsort reads the parent's first row for a contiguous sliced view with a nonzero data offset. This returns {{0, 1, 2}} below, while the documented sorted-order contract requires {{2, 1, 0}} for the selected values {3, 2, 1}. xt::strided_view has the same problem.

xt::xarray<int> a = {{1, 2, 3}, {3, 2, 1}};
auto v = xt::view(a, xt::range(1, 2), xt::all());
auto indices = xt::argsort(v);

We add the input's data_offset() when starting leading-axis traversal.

We verified that the regression fails before the change and passes afterward alongside the existing sorting tests.

— ChipLab