Nan gradient for elu and selu.

[racinmat]

julia> using Zygote
julia> gradient(x -> sum(Zygote.selu.(x)), [1_000.0])
([1.0507009873554805],)
julia> gradient(x -> Zygote.selu(x), 1_000.0)
(NaN,)
julia> gradient(x -> sum(Zygote.elu.(x)), [1_000.0])
([NaN],)
julia> gradient(x -> Zygote.elu(x), 1_000.0)
(NaN,)

the broadcasted version for Selu works because of https://github.com/FluxML/Zygote.jl/blob/master/src/lib/nnlib.jl#L17
but I assume non-broadcasted version should be added and the elu derivation should also be added, right?

[DhairyaLGandhi]

+1 for this

[racinmat]

Created PR but it would be great if someone reviewed if it's correct.

[racinmat]

Closed by #765