Wrong Fill'ed gradient

[cossio]

julia> Zygote.gradient(x -> sum(x .* randn.()), randn(5))
(Fill(-0.30782754826253156, 5),)

This is wrong. The result should be the random vector of coefficients.

[mcabbott]

Better example, I think:

julia> Zygote.gradient(x -> sum(abs, x .* randn.()), ones(5))
([2.366255991572848, 2.366255991572848, 2.366255991572848, 2.366255991572848, 2.366255991572848],)

julia> Zygote.gradient(x -> sum(x .* randn.()), ones(5))
(Fill(0.16740972975253307, 5),)

From FillArrays's readme:

julia> broadcast(_ -> rand(), Fill("pi", 2,5))
2×5 Fill{Float64}, with entries equal to 0.6010457938025311

julia> ones(1,5) .+ (_ -> rand()).(Fill("vec", 2))
2×5 Matrix{Float64}:
 1.83695  1.83695  1.83695  1.83695  1.83695
 1.83695  1.83695  1.83695  1.83695  1.83695

julia> Fill(1.0, 2, 5) .+ rand.()  # but this is not altered
2×5 Matrix{Float64}:
 1.50444  1.16431  1.55788  1.70783  1.00847
 1.61302  1.968    1.6408   1.02243  1.18068