Compile the inner fct in scan only once per instance.

theano/scan_module/scan_op.py

@@ -566,6 +566,9 @@ class Scan(PureOp):
                 profile = ScanProfileStats(name=self.name)
         elif self.profile:
             profile = self.profile
+        # make_thunk can be called many times on the same op
+        # we do not want to recompile the inner fct every time.
+        if not getattr(self, 'fn', None):
             self.fn = function(wrapped_inputs,
                                wrapped_outputs,
                                mode=self.mode_instance,