R operator for the join op

theano/tensor/basic.py

@@ -4011,6 +4011,11 @@ class Join(Op):
         out[0] = theano._asarray(numpy.concatenate(tensors, axis = axis),
                 dtype=node.outputs[0].type.dtype)
 
+    def R_op(self, inputs, eval_points):
+        if None in eval_points[1:]:
+            return [None]
+        return self.make_node(inputs[0], *eval_points[1:]).outputs
+
     def grad(self, axis_and_tensors, grads):
         """ The gradient wrt a join op is a `Split`, used to partition the gradient along the
         `axis` which was used for joining.