More optimizations

• Speedup backprop
• Fix gpu backend for different CUDA versions

juno_samples/matmul/src/matmul.sch

@@ -51,7 +51,7 @@ if feature("cuda") {
   fork-coalesce(*);
   infer-schedules(*);
   dce(*);
-  rewrite(*);
+  //rewrite(*);
   fixpoint {
     simplify-cfg(*);
     dce(*);