Pass size to HalfNormal random method (#3686)

* Pass size to HalfNormal random method

* Add note in release notes

Author: ColCarroll

RELEASE-NOTES.md

@@ -31,7 +31,7 @@
 - Fixed a defect in `OrderedLogistic.__init__` that unnecessarily increased the dimensionality of the underlying `p`. Related to issue issue [#3535](https://github.com/pymc-devs/pymc3/issues/3535) but was not the true cause of it.
 - Wrapped `DensityDist.rand` with `generate_samples` to make it aware of the distribution's shape. Added control flow attributes to still be able to behave as in earlier versions, and to control how to interpret the `size` parameter in the `random` callable signature. Fixes [3553](https://github.com/pymc-devs/pymc3/issues/3553)
 - Added `theano.gof.graph.Constant` to type checks done in `_draw_value` (fixes issue [3595](https://github.com/pymc-devs/pymc3/issues/3595))
-
+- `HalfNormal` did not used to work properly in `draw_values`, `sample_prior_predictive`, or `sample_posterior_predictive` (fixes issue [3686](https://github.com/pymc-devs/pymc3/pull/3686))
 
 ## PyMC3 3.7 (May 29 2019)
 

pymc3/distributions/continuous.py

@@ -855,7 +855,7 @@ def random(self, point=None, size=None):
         -------
         array
         """
-        sigma = draw_values([self.sigma], point=point)[0]
+        sigma = draw_values([self.sigma], point=point, size=size)[0]
         return generate_samples(stats.halfnorm.rvs, loc=0., scale=sigma,
                                 dist_shape=self.shape,
                                 size=size)