Add an exception for old argmax nodes.

theano/tensor/basic.py

@@ -1462,6 +1462,10 @@ class Argmax(Op):
         outputs = [tensor('int64', broadcastable, name='argmax')]
         return Apply(self, inputs, outputs)
 
+    def prepare_node(self, node):
+        if len(node.inputs) == 2:
+            raise ValueError('You are trying to compile a graph with an old Argmax node.  Either reoptimize your graph or rebuild it to get the new node format.')
+
     def perform(self, node, inp, outs, params):
         x, = inp
         axes = self.axis