Bug Description

Title.

Steps to Reproduce

import numpyro.distributions as dist

assert jnp.isnan(dist.Beta(1.0, 8.0).log_prob(0.))

Expected Behavior

assert jnp.isclose(dist.Beta(1.0, 8.0).log_prob(0.), 2.079)

The log_prob implementation should handle the cases with any concentration parameter equal to 1. See for example the Julia implementation.