Groupdot

Hello,

This includes the tests for groupdot and groupdotgrad in sandbox/cuda/tests/test_groupdot.py.
This branch also has the codes that you had submitted as PR to main theano, with a couple of slight modifications. IIRC, these are the two changes: 
1. b.setValue in groupdotgrad's perform
2. change the dtype of grad_on_out in make_thunk