Make an opt don't add useless node that futur opt would remove.

theano/tensor/opt.py

@@ -3251,12 +3251,15 @@ def local_IncSubtensor_serialize(node):
         if movable_inputs:
             new_inputs = ([i for i in node.inputs if not movable(i)] +
                           [mi.owner.inputs[0] for mi in movable_inputs])
-            new_add = T.add(*new_inputs)
+            if len(new_inputs) == 0:
+                new_add = new_inputs[0]
+            else:
+                new_add = T.add(*new_inputs)
 
-            # Copy over stacktrace from original output, as an error
-            # (e.g. an index error) in this add operation should
-            # correspond to an error in the original add operation.
-            copy_stack_trace(node.outputs[0], new_add)
+                # Copy over stacktrace from original output, as an error
+                # (e.g. an index error) in this add operation should
+                # correspond to an error in the original add operation.
+                copy_stack_trace(node.outputs[0], new_add)
 
             # stack up the new incsubtensors
             tip = new_add