- now use explicit NullType() or DisconnectedType() instance for
  OpFromGraph special types. 'default' for no gradient/Rop override
- connection pattern now considers gradient override
- revert hackish changes done to theano/gradient.py
- bug fix for theano.gof.graph.io_connection_pattern with isolated output
- bug fix when _recompute_grad/rop_op is called twice for OfG instance