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)
  • planar and radial flow
  • Real NVP
  • Neural Spline flow
  • Sparse Hamiltonian flow

[yebai]

@zuhengxu, can you push to merge this PR if nothing significant is standing? Thanks!

[zuhengxu]

No prob! I'll get back to this PR this weekend and hopefully merge it ASAP.

[yebai]

Could we add a script to test whether all the example models run, e.g. on buildkite GPU?

[zuhengxu]

Yes, let me clean up the Ham VI demo and then add the test script. Currently I can confirm that all the examples run on CPUs by doing

include("demo_..._fllow.jl")

[zuhengxu]

Could we add a script to test whether all the example models run, e.g. on buildkite GPU?

@yebai Currently all the examples are only tested on CPUs. Do you have examples of such script (for testing runs) that I can refer to? (I did do include(...jl) manually for all the demos though.)

In terms of GPU, shall we first merge this PR and do GPU examples/test (#49) on a separate PR? the purpose this PR is just to demonstrate how to define and customize flow layers (at least for those popular flows). I think that to properly take advantage of GPU, we might want some interface adjustment.