Merge #1231

1231: use `ntuple` in conv r=MikeInnes a=MikeInnes

This is the right abstraction over `map`, and in particular is a bit easier to compile away in some cases. 

As this is a trivial change from Flux's perspective it's not easy to test here, but there are downstream tests in XLA.jl.

Co-authored-by: Mike J Innes <mike.j.innes@gmail.com>

src/layers/conv.jl

@@ -132,7 +132,7 @@ end
 function (c::Conv)(x::AbstractArray)
   # TODO: breaks gpu broadcast :(
   # ndims(x) == ndims(c.weight)-1 && return squeezebatch(c(reshape(x, size(x)..., 1)))
-  σ, b = c.σ, reshape(c.bias, map(_->1, c.stride)..., :, 1)
+  σ, b = c.σ, reshape(c.bias, ntuple(_->1, length(c.stride))..., :, 1)
   cdims = DenseConvDims(x, c.weight; stride=c.stride, padding=c.pad, dilation=c.dilation)
   σ.(conv(x, c.weight, cdims) .+ b)
 end