Unnecessary to update the shape parameter in blocked sampler?

[juanfung]

From discussion in #23:

Note that the shape parameter for sampling alpha should not change

a_star = input.priors.prior_dp.alpha_shape + input.priors.prior_dp.J - 1

since alpha_shape is the prior shape parameter (hence fixed), J is the number of components at which the stick-breaking process is truncated (hence fixed).

But is this correct?