Add `CategoricalMADE`

[jnsbck]

What does this implement/fix? Explain your changes

This implements a CategoricalMADE to generelize MNLE to multiple discrete dimensions addressing #1112. Essentially adapts nflows's MixtureofGaussiansMADE to autoregressively model categorical distributions.

Does this close any currently open issues?

Fixes #1112

Comments

I have already discussed this with @michaeldeistler.

Checklist

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