perf/sgd: use GPU for computation of weight updates
perf/sgd: use GPU for computation of weight updates

Actually SGD now uses backend it was instantiated with. Before this patch it
used hardcoded `Native` backend.

On `leaf-examples mnist` and GTX 960 this provides 2.4x performance increase.

CLOSE: 88

autumnai/leaf,ratpoison-io/leaf,autumnai/leaf,ratpoison-io/leaf