Fix models not rendering after application of the equalize effect handler

[BenZickel]

Problem

After using the pyro.poutine.equalize effect handler model rendering with pyro.render_model does not work due to no base_dist attribute in Delta sampled sites.

Solution

Add masking so that the sampling function has the base_dist attribute as expected by pyro.render_model for Delta sampled sites.

[BenZickel]

Also added a fix for this lint failure due to a mypy update.