Hi.

I would like to compute the Hessian matrix of my Flux model.

I tried something like this without success:

```
n = 100
X = rand(2, n)
Y = rand(1, n)
```

```
model = Chain(Dense(2,2),Dense(2,1))
loss(x,y) = Flux.mse(model(x),y)
hess = Zygote.hessian(Flux.params(model)) do
loss(X, Y)
end
```

```
ERROR: MethodError: no method matching hessian(::var"#129#130", ::Params)
Closest candidates are:
hessian(::Any, ::AbstractArray) at /user/mpoliti/home/.julia/packages/Zygote/ggM8Z/src/lib/utils.jl:113
Stacktrace:
[1] top-level scope at REPL[281]:1
```