Project nothing to nothing

[DhairyaLGandhi]

Ref SciML/SciMLSensitivity.jl#493

[DhairyaLGandhi]

This is likely to break some tests, but seems like the cogent thing to do for some cases.

[oxinabox]

I am not sure if this is the right place for this fix.
Or if we need a more general insertion of the code to convert CRC types in to Zygote types.
I think I saw it happening with other types than just Nothing.

@mcabbott should review.

[DhairyaLGandhi]

Since _project is the last transform Zygote makes now, it should generically output Zygote types. That way, we need a conversion layer between _project and ProjectTo to go from CR types to Zygote's.

[mcabbott]

This seems completely safe to me. Although might be covered by the AbstractZero line above?

Re PR's original change c53df99 to propagate nothings, I was surprised this broke no tests. I don't recall now what test motivated that method in the first place, and it's possible that some later change removed the need.

Does this solve SciML/SciMLSensitivity.jl#493? As noted there, I can't reproduce this locally.

[DhairyaLGandhi]

ref SciML/SciMLSensitivity.jl#493 (comment)

I believe we have the _project in the getindex adjoint setup to handle OneElement which doesn't have information about the type of the array. Why do we have OneElement? Our accumulation speedup came from generalising accum, and incrementally through OneElement. Judging by the few number of times it coming up in issues, it might be that we can have a more stable and simpler implementation by rethinking it somewhat.

[CarloLucibello]

closing as stale, possibly also not needed anymore