more NF examples

[zuhengxu]

as per issue #15

• [ ] implement a set of famous NFs illustrating the usage of NormalizingFlows.jl + Bijectors.jl (a tentative list)
  • [x] planar and radial flow
  • [x] Real NVP
  • [x] Neural Spline flow
  • [ ] Neural Autoregressive Flows
  • [x] Sparse Hamiltonian flow
  • [x] Hamiltonian VAE