Distribution wrappers in handlers.pyro should be deprecated in favor of defterm

[eb8680]

It should be possible to replace PositionalDistribution with defterm and defdata implementations for TorchDistributions. The same things would also serve as a replacement for NamedDistribution, though we'd need an extra Operation there since the basic Distribution API doesn't support __getitem__ or an equivalent.

[eb8680]

#214 makes some progress toward this but doesn't yet fully resolve it.

I think the general solution would be a pair of operations bind_dims and unbind_dims in handlers.torch that subsume handlers.torch.to_tensor and positional_distribution/named_distribution from #214:

T = TypeVar("T", bound=torch.Tensor | torch.distributions.Distribution | tree.Structure[torch.Tensor | torch.distributions.Distribution)

@defop
def bind_dims(
  value: Annotated[T, Scoped[A | B]], 
  *names: Annotated[Operation[[], int], Scoped[B]]
) -> Annotated[T, Scoped[A]]:
  ...

@defop
def unbind_dims(
  value: Annotated[T, Scoped[A]],
  *names: Annotated[Operation[[], int], Scoped[B]]
) -> Annotated[T, Scoped[A | B]]:
  ...

bind_dims is basically the same as our current to_tensor extended with logic for handling call. unbind_dims is a little trickier because it shouldn't distribute over operations that bind positional dimensions.

[eb8680]

Removing from the milestone since #214 is sufficient for now and the general solution would be quite a bit more effort.