support arbitrary number of batch dimensions in batched_mul

[CarloLucibello]

Motivation and description

use case is the multihead attention layer in FluxML/Flux.jl#2146

Possible Implementation

function NNlib.batched_mul(x::AbstractArray{T1,N}, y::AbstractArray{T2,N}) where {T1,T2,N}
  sz = size(x)[3:end]
  @assert sz == size(y)[3:end]
  x2 = reshape(x, size(x, 1), size(x, 2), :)
  y2 = reshape(y, size(y, 1), size(y, 2), :)
  z = NNlib.batched_mul(x2, y2)
  return reshape(z, size(z, 1), size(z, 2), sz...)
end

[mcabbott]

This is #391