axis_slice_iterator and axis_iterator do not function with xtensors

[dchansen]

axis slice iterator is broken when used with xtensors (godbolt) and axis_iterator simply does not compile.

This issue for both cases is that the strides_type and shape_type in axis_slice_iterator and axis_iterator are taken from the given xexpression, which is not correct for xtensor as the resulting strided_view should anyhow be dynamically sized.

The alternative would be to assign a dynamic strides and shape_type, but it was not clear to me what the correct replacement type should be.

[Stef-Sijben]

This appears to be a duplicate of #2116.

[spectre-ns]

The godbolt example appears to compile in all 3 major compilers.

[dchansen]

Yes - but they also produce the wrong output (see the output tab). The shape reported is (2, 0, 0) in this example

[spectre-ns]

Yes - but they also produce the wrong output (see the output tab). The shape reported is (2, 0, 0) in this example

Ahh... Yes, I see that now.