add convenient functions stack/unstack

[johnnychen94]

Unlike Align/Slices, the input to stack/unstack is the outer dimensions. It's possible to support multiple dimensions, but I didn't bother here because my direct target is one-dimensional case.

@DhairyaLGandhi this is a direct replacement for Flux.stack/Flux.unstack without allocating memories.

[DhairyaLGandhi]

How does it behave with AD?

[johnnychen94]

Good question. Without even trying it, I bet it doesn't work here. I was just thinking about the data preparation stage stack(imgs, 4) when I'm writing the image data layout blog for Flux.

This package is pretty lightweight so I believe @bramtayl won't want to have extra dependencies here to support AD.

[DhairyaLGandhi]

Right, I tend to agree. I know that for LazyArrays, we had to support the apply! function which was the simple API that all of LazyArrays exposed. If JuliennedArrays has a similar format, we can add that somewhere too to keep the package light and AD ready.

[DhairyaLGandhi]

To be clear, it might still work (LazyArrays did at the time) but the adjoint meant that it remained lazy in the backwards pass as well, so it was needed for efficiency.

[johnnychen94]

I've opened another PR #28 for test updates and enhancements, thus 1b607d8 is unnecessary now.

[bramtayl]

Hi! I'm glad you find the package useful. I'd be very happy to accept a PR for auto-differentiation (although I'm not sure I could understand it enough to review it). I'm not sure I understand why stack/unstack are needed though?

[bramtayl]

Also, on the AD part, have you seen this? https://github.com/mcabbott/SliceMap.jl