Cauchy Distribution (#13894)

* cauchy docstring

* added __init__ method

* added other cauchy methods with non-nan outcomes

* survival functions, nan stats

* log prob, shape tests

* tests for other deterministic methods

* sample tests

* shape, invalid arg tests

* pylint fixes

* registering class to allowed symbols

* corrections to examples in cauchy docstring

* cauchy with softplus scale

* tested, registered cauchy w softplus scale

* review changes, log1p where possible, math to np

* pylint fixes, remove references to CauchyWithSoftplusScale

* use np.nan, not float("nan") for nan_stats

* Update cauchy.py

* Corrected cauchy test path in BUILD

* tests fixes