Check that torch is available, error otherwise

Usage

require_torch(
  what = "This density estimator",
  alternative = paste("Alternatively use", "density_estimator =",
    "'linear_gaussian' for a", "torch-free baseline.")
)

Arguments

what: What needs torch, as the subject of the sentence. The default suits the density estimators, which is where most of the calls are.
alternative: The torch-free thing to do instead, as a full sentence. Every caller has one, and naming it is the difference between a dead end and a next step.