ChainRules.jl

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This causes problems with Zygote:

julia> using Zygote

julia> gradient(x -> sum([x x]), pi/2)
(2.0,)

julia> gradient(x -> sum(Float32[x x]), pi/2)
ERROR: Mutating arrays is not supported -- called setindex!(Matrix{Float32}, ...)
...
  [7] typed_hcat
    @ Zygote ./abstractarray.jl:1610 [inlined]

Ideally we'd teach ChainRules how to handled this statement without going through to the implementation.

See also #695