abstract type NetworkPrior end
All priors over the network parameters are subtypes of NetworkPrior.
num_params_hyper: Number of hyper priors (only those that should be inferred)
Each NetworkPrior must be callable in the form of
θnetis a vector of network parameters
θhyperis a vector of hyper parameters
The return value must be the logprior including all hyper-priors.
Each NetworkPrior must also implement a sample function returning a vector
θnet of network parameters drawn from the prior.