make shared array constructor consistent to array constructor

[Roger-luo]

I'm not sure if this init is still needed? can we just provide it with fill interface?

Why do we need a init function in the constructor?

• use undef for uninitialized SharedArray, but preserve the init keyword (you need it for files)
• similar should return SharedArray

[StefanKarpinski]

Not sure who the best person to review SharedArray code is. @mbauman do you know? This looks fairly straightforward to me but I'm not certain.

[mbauman]

Unfortunately the similar part here was tried before (#12560) but it ran into many difficulties in practice and ended up getting reverted (#12964).

[Roger-luo]

@mbauman What about return SharedArray for similar and have a new method that return a similar data?

So in my case, I tried to implement something with generic array type, which allows me to run either on multiprocess with SharedArray shares the data, or on a single process with simple Array, CuArray, etc. And then I'll need some consistent method to construct the array.

I currently have to work around this like:

_similar(xs...; kwargs...) = similar(xs...; kwargs...)

_similar(S::SharedArray, T::Type, dims::Dims{N}; pids=S.pids) where N = SharedArray{T, N}(dims; pids=pids)
_similar(S::SharedArray{ST, N}, T::Type; pids=S.pids) where {ST, N} = _similar(S, T, size(S); pids=pids)
_similar(S::SharedArray, dims::Dims; pids=S.pids) = _similar(S, eltype(S), dims; pids=pids)
_similar(S::SharedArray; pids=S.pids) = _similar(S, eltype(S), size(S); pids=pids)