At the moment PyMC3 cannot do this automatically and users have to choose a compound distribution if they wish to exploit conjugacy (eg. with the BetaBinomial or Dirichlet Multinomial distributions).
Outside of a few compound distributions that are already implemented, the user would have to write his own if they wished to exploit this.
Automatic reparametrization is something we probably want to implement somewhere down the road, but we still need to implement the compound distributions.
On the other hand NUTS is often fast enough at sampling multiple variables that conjugacy is not so critical.