Limit model graph nodes to named model variables

Some nodes such as `NoneConst` in an Aesara graph are named, but aren't model variables
that the user is interested in.

Author: michaelosthege

pymc/model_graph.py

@@ -47,7 +47,12 @@ def _expand(x):
                 return reversed(x.owner.inputs)
             return []
 
-        parents = {get_var_name(x) for x in walk(nodes=var.owner.inputs, expand=_expand) if x.name}
+        parents = {
+            get_var_name(x)
+            for x in walk(nodes=var.owner.inputs, expand=_expand)
+            # Only consider nodes that are in the named model variables.
+            if x.name and x.name in self._all_var_names
+        }
 
         return parents