* number fo threads on GPU must be a multiple of NUM_VECTOR_OP_THREADS_PER_BLOCK
* this means we will have more threads on the GPU than the number of streams we
actually want.
* the fix involves making these extra threads do nothing, instead of corrupting
the stream of random samples !

TODO: review MRG_RandomStreams test suite.

implementations (on CPU). DebugMode already takes care of this.

random numbers properly. This was hiding a GPU bug. Fix is in progress.

changed an assert to an Exception so that if the optimization fails it doesn't bring down the whole process

detect at run time the gpu pointeur size and int size. Use that to allow fusing more gpu elemwise together