Add Padding to Convolutions

[avik-pal]

Would it be possible to add padding support to convolutions? My current application relies on keeping the input size fixed but uses a very small model (see below). (Without padding SimpleChains outperforms Lux by >30x on these workloads)

model = Chain(
       Conv((11,), 1 => 16, Lux.relu; pad=Lux.SamePad()),
       Conv((11,), 16 => 1; pad=Lux.SamePad()),
)

cc @ChrisRackauckas (this is for the learned solvers work)