Test Exponential distribution in test_find_constrained_prior

Closes #5289

Author: ricardoV94

pymc/tests/test_func_utils.py

@@ -25,8 +25,7 @@
         (pm.Normal, 155, 180, {"mu": 170, "sigma": 3}, {}),
         (pm.StudentT, 0.1, 0.4, {"mu": 10, "sigma": 3}, {"nu": 7}),
         (pm.StudentT, 0, 1, {"mu": 5, "sigma": 2, "nu": 7}, {}),
-        # (pm.Exponential, 0, 1, {"lam": 1}, {}), PyMC Exponential gradient is failing miserably,
-        # need to figure out why
+        (pm.Exponential, 0, 1, {"lam": 1}, {}),
         (pm.HalfNormal, 0, 1, {"sigma": 1}, {}),
         (pm.Binomial, 0, 8, {"p": 0.5}, {"n": 10}),
         (pm.Poisson, 1, 15, {"mu": 10}, {}),