add support for undef as override parameter

theano/compile/builders.py

@@ -9,6 +9,7 @@ from theano.compile.function_module import orig_function
 from theano.compile import SharedVariable, rebuild_collect_shared, optdb
 from theano.gof import ops_with_inner_function
 from theano.gof.graph import io_connection_pattern
+from theano.gof.utils import undef
 
 
 class OpFromGraph(gof.Op):
@@ -28,13 +29,14 @@ class OpFromGraph(gof.Op):
     inline: bool, optional
         if True, will cause the Op's original graph being used during
         compilation, otherwise will use a pre-compiled function inside.
-    grad_overrides: None | function | list of (None|function), optional
+    grad_overrides: None | undef | function | list of (None|undef|function), optional
         Used to override default gradient routine.
         Overriding function(s) must take two list of variable(s) as inputs,
         the original inputs and ups gradients
         For different `grad_overrides`:
 
         - `None` : will use default gradient routine.
+        - theano.utils.undef : No gradient will be used (zero)
         - function : must return list of Variable.
         - list : each function must return a single Variable. The order
             of the list must corresponds to inputs
@@ -196,9 +198,19 @@ class OpFromGraph(gof.Op):
                 wrt=wrt_l,
                 disconnected_inputs='ignore') if wrt_l else [])
             # combine overriding gradients
+            all_grads_l = []
+            for inp, gov in izip(self.local_inputs, goverrides_l):
+                if gov is None:
+                    all_grads_l.append(next(gdefaults))
+                elif gov is undef:
+                    all_grads_l.append(
+                        inp.zeros_like().astype(theano.config.floatX))
+                else:
+                    all_grads_l.append(gov(self.local_inputs, output_grads))
+        elif self._grad_op is undef:
             all_grads_l = [
-                gov(self.local_inputs, output_grads) if gov
-                else next(gdefaults) for gov in goverrides_l]
+                inp.zeros_like().astype(theano.config.floatX)
+                for inp in self.local_inputs]
         else:
             all_grads_l = self._grad_op(self.local_inputs, output_grads)
         self._grad_op = type(self)(
@@ -227,6 +239,8 @@ class OpFromGraph(gof.Op):
     def grad(self, inputs, output_grads):
         if not self._grad_op_is_cached:
             self._recompute_grad_op()
+        if self._grad_op is undef:
+            return [None for _ in self.input_types]
         return self._grad_op(*(list(inputs) + list(output_grads)))
 
     def make_node(self, *inputs):