Remove cached plates from AutoGuide.

[tillahoffmann]

The plate primitive may leak tracers through its _indices attribute if created outside a pure function. The AutoGuide implementation caches plates in a plates attribute which results in a tracer leak when the guide is called (see example below).

This PR removes the caching such that calls to the guide are pure.

>>> import jax
>>> import numpyro


>>> def model():
...     with numpyro.plate("n", 2):
...         numpyro.sample("x", numpyro.distributions.Normal())

>>> guide = numpyro.infer.autoguide.AutoDelta(model)

>>> numpyro.handlers.seed(guide, 9)()
>>> guide.plates["n"]._indices
Array([0, 1], dtype=int32)

>>> jax.jit(lambda: numpyro.handlers.seed(guide, 9)())()
>>> guide.plates["n"]._indices
Traced<ShapedArray(int32[2])>with<DynamicJaxprTrace>

[fehiepsi]

Thanks, Till!