Half Normal Distribution (and inverse error function) (#14056)

* foldednormal docstring

* folded __init__ method

* prob, log_prob methods

* rewrote halfnormal docstring

* initial implementation of dist methods

* halfnormal unit tests

* registered HalfNormal to contrib.distributions

* added erfinv function

* unit tests for erfinv

* registered erfinv symbol

* cdf, pdf now deal with x < 0 correctly

* pylint fixes

* cuda_py test reference in BUILD

* erfinv fixes

* corrections to scipy reference tests

* Added reference to entropy test case.