simplify handling of known_grads

theano/gradient.py

@@ -438,6 +438,9 @@ def grad(cost, wrt, consider_constant=None,
     # build a dict mapping var to the gradient of cost with respect to var
     grad_dict = {}
 
+    if known_grads is None:
+        known_grads = {}
+
     # The gradient of the cost is 1 unless specified otherwise by known_grads.
     if cost is not None:
         if cost in known_grads:
@@ -457,7 +460,6 @@ def grad(cost, wrt, consider_constant=None,
 
         grad_dict[cost] = g_cost
 
-    if known_grads is not None:
     for var in known_grads:
         g_var = known_grads[var]