Preserve 0d arrays in Zygote.accum

[lxvm]

Zygote.accum mirrored an old Julia bug that got fixed by Julia 1.6, which is the version supported by Zygote. This pr changes the behavior to match Julia's

PR Checklist

• Tests are added
• Documentation, if applicable

[mcabbott]

Do you have an example of user-facing code which triggers this?

julia> gradient(x -> first(x) + sum(x), fill(1.0))[1]
0-dimensional Array{Float64, 0}:
2.0

[lxvm]

Yes, it's #1490

[darsnack]

We discussed this on call and agree with the change. The behavior of accum should match x + y not x .+ y.

[lxvm]

Great, I've rebased this on master so it should be ready to go