Simple memory tools

[MikeInnes]

e.g.

julia> m = Chain(Dense(10, 5, relu), Dense(5, 2)) |> gpu
Chain(Dense(10, 5, NNlib.relu), Dense(5, 2))

julia> y = m(gpu(rand(10)))
Tracked 2-element CuArray{Float32,1}:
 0.14124388f0
 1.0137802f0

julia> Tracker.mem(y)
511

This is pretty coarse grained for now (Zygote already has something much more advanced).

[bhvieira]

Does Tracker have this functionality right now? The PR could still be moved there

[MikeInnes]

I don't think I'm going to have time to look at Tracker stuff. So I'll close for now, though someone could take this up if they want to.