Use gradient_undefined instead of None in MRG uniform
Catch exception and forward NullType to input gradients, instead of failing
Try to detect when gcc incorrectly identifies processors as -avx
Copy GpuSubtensor to the new back-end, enable its test.
Add the opt to move IncSubtensor to the new GPU back-end.
GpuIncSubtensor continued, not finished.
A first working implementation of GpuIncSubtensor.perform()

As GpuArray don't implement += and +, we pass by another interface.
remove fct not used anymore.
Advance GpuIncSubtensor c code
Finish GpuIncSubtensor c code.
Don't include Python/NumPy include, convert the string.

Otherwise, in some platform the include fail. This is more robust.
remove the import of pycuda in the new back-end.
Fix pickling of GpuIncSubtensor
Try to fix how Null gradients are propagated in Scan

- propagate NullType when summing different gradient contributions
- identify which outer outputs of the scan node implementing the grad
  should be Null
- replace them, and the corresponding inner variables, with zeros
  of the appropriate size, so the scan node can be compiled and run.
- introduce a difference in undefined gradient because going through
  a shared variable with updates, and because a NullType was returned
  by the internal graph.
Actually compile and run the grad scan node.
Update comment, indentation.
Updated OpFromGraph docstring.
Raise error if the user try to use shared variable.
Fix error msg.
Remove OpFromGraph grad_depth parameter.