de_log_prob()/de_sample() need to know where to put their input
tensors, and the one place that is always true is the net itself –
de$device (set at fit time) is a record of that, not a guarantee, since
nothing stops a user from moving the net with net$to() afterwards. A net
with no parameters (there is no such estimator today, but nothing rules one
out) defaults to CPU.