Add fallback for missing value_var in BaseTrace

Author: brandonwillard

pymc3/backends/base.py

@@ -61,7 +61,7 @@ def __init__(self, name, model=None, vars=None, test_point=None):
         model = modelcontext(model)
         self.model = model
         if vars is None:
-            vars = [v.tag.value_var for v in model.unobserved_RVs]
+            vars = [getattr(v.tag, "value_var", v) for v in model.unobserved_RVs]
         self.vars = vars
         self.varnames = [var.name for var in vars]
         self.fn = model.fastfn(vars)