GpuElemwise now raise an error when we could broadcast input to make the code run but did not set the broadcast flags to True. Added test that we raise this error.

This allow to fuse more op in GpuElemwise(gpu have a limit in the number of fct inputs) and make the graph a little simpler.

Make easier the transition to MRG_RandomStreams from code using RandomStreams
or SharedRandomStreams.

make the EquilibriumOptimizer a little faster by not triing to optimize node that have been removed.

modif canonicaly to lift toward input abs throught mul and true_div. This allow the canonicalizer simplier check_for_x_over_absX to cover more case.

added a mathematical definition of Op.grad

 some fix for scan when computing gradients wrt to shared variables that where not passed to the inner function (added a new test in test_scan.py