Make Delta.log_prob jit-able on metal and raise explicit error for Delta sampling site during initialization.

[tillahoffmann]

Two Delta distribution related changes.

1. The current implementation uses jnp.log(value == self.v), where self.v is the value of the Delta distribution. This is not jit-able on metal (cf. lambda x: jnp.log(x == 1) is not jit-able on metal. jax-ml/jax#25935). The updated implementation uses jnp.where(value == self.v, 0, -jnp.inf) which is equivalent but jit-able.
2. Delta sampling sites lead to a non-specific error message RuntimeError: Cannot find valid initial parameters. Please check your model again.. This change adds a line to check for unobserved Delta sites in a model and raises a more explicit error.

[fehiepsi]

Thanks, @tillahoffmann!