Implement mix discrete and continuous sampling

[kazewong]

In some use cases, the users may want to switch between discrete sampling and continuous sampling.

There are two parts to this problem:

1. Let the user defining an objective function that support both continuous and discrete variables.
2. Sample the variables.

The second step can be done with some thing like
https://num.pyro.ai/en/latest/_modules/numpyro/infer/hmc_gibbs.html#DiscreteHMCGibbs