fixed OpFromGraph's handling of disconnected inputs

theano/compile/builders.py

@@ -55,8 +55,12 @@ class OpFromGraph(gof.Op):
 
         if grad_depth > 0:
             output_grads = [t() for t in self.output_types]
+            # OpFromGraph doesn't implement a connection_pattern, so for now we regard
+            # all inputs and outputs as connected. This will compute the right numerical
+            # value for the gradients but could fail to raise the disconnected inputs error
+            # in some cases.
             gs = G.grad(cost=None, known_grads=dict(zip(self.outputs, output_grads)),
-                    wrt=self.inputs)
+                    wrt=self.inputs, disconnected_inputs='ignore')
             self.grad_ops = []
             for g in gs:
                 if g is None: