Make sure shared variables have correct broadcasting in ValueGradFunction

Author: brandonwillard

pymc3/model.py

@@ -427,7 +427,9 @@ def __init__(
         givens = []
         self._extra_vars_shared = {}
         for var, value in extra_vars_and_values.items():
-            shared = aesara.shared(value, var.name + "_shared__")
+            shared = aesara.shared(
+                value, var.name + "_shared__", broadcastable=[s == 1 for s in value.shape]
+            )
             self._extra_vars_shared[var.name] = shared
             givens.append((var, shared))