Allow transfer type to hardcode the output dtype

theano/scalar/basic.py

@@ -724,7 +724,7 @@ def same_out_float_only(type):
 
 class transfer_type(gof.utils.object2):
     def __init__(self, *transfer):
-        assert all(type(x) == int for x in transfer)
+        assert all(type(x) in [int, str] or x is None for x in transfer)
         self.transfer = transfer
 
     def __str__(self):
@@ -736,6 +736,8 @@ class transfer_type(gof.utils.object2):
         for i in self.transfer:
             if i is None:
                 retval += [upcast]
+            elif isinstance(i, str):
+                retval += [i]
             else:
                 retval += [types[i]]
         return retval
@@ -3410,7 +3412,10 @@ class Composite(ScalarOp):
                 return lambda inputs: r.data
             node = r.owner
             producers = [compose_impl(input) for input in node.inputs]
-            return lambda inputs: node.op.impl(*[p(inputs) for p in producers])
+
+            def f(inputs):
+                return node.op.impl(*[p(inputs) for p in producers])
+            return f
         self._impls = [compose_impl(r) for r in self.fgraph.outputs]
 
     def init_name(self):