Consider deprecating `DeterministicVariable` in favor of Delta distribtions

[damonbayer]

I think we can use numpyro.distributions.Delta in all cases where we currently use our own DeterministicVariable, which would reduce the complexity of the codebase.

>>> import jax.numpy as jnp
>>> import numpyro
>>> from pyrenew.deterministic import DeterministicVariable
>>> 
>>> val = jnp.array([1, 2, 3])
>>> det_var = DeterministicVariable("my var", val)
>>> delta_dist = numpyro.distributions.Delta(val)
>>> 
>>> delta_dist.sample(key = None)
Array([1, 2, 3], dtype=int32)
>>> det_var.sample()
Array([1, 2, 3], dtype=int32)