chex: alias PRNGKey to jax.Array

Going forward, `jax.Array` is the correct type annotation for both new-style and old-style PRNG keys in JAX (see google/jax#17297)

Note that `jax.random.KeyArray` has been aliased to `Any` under TYPE_CHECKING, and so this change will make existing annotations more strict, not less strict.

PiperOrigin-RevId: 565133147

distrax/_src/distributions/distribution.py

@@ -324,7 +324,7 @@ def convert_seed_and_sample_shape(
   else:  # key is of type PRNGKey
     rng = seed
 
-  return rng, sample_shape
+  return rng, sample_shape  # type: ignore[bad-return-type]
 
 
 def to_batch_shape_index(