More optimizations

• Speedup backprop
• Fix gpu backend for different CUDA versions

juno_samples/rodinia/bfs/src/cpu.sch

@@ -46,8 +46,6 @@ let traverse_body = outline(inner);
 let (outer, inner) = fork-reshape[[1], [0]](collect);
 let collect_body = outline(inner);
 
-let init_body = init;
-// Following code seems to generate breaking RT code
 fork-tile[32, 0, false, true](init);
 let (outer, inner) = fork-reshape[[1], [0]](init);
 let init_body = outline(inner);