fix half conversion (#280)

Author: rusty1s

csrc/cuda/utils.cuh

@@ -9,11 +9,11 @@
 __device__ __inline__ at::Half __shfl_up_sync(const unsigned mask,
                                               const at::Half var,
                                               const unsigned int delta) {
-  return __shfl_up_sync(mask, (__half)var, delta);
+  return __shfl_up_sync(mask, var.operator __half(), delta);
 }
 
 __device__ __inline__ at::Half __shfl_down_sync(const unsigned mask,
                                                 const at::Half var,
                                                 const unsigned int delta) {
-  return __shfl_down_sync(mask, (__half)var, delta);
+  return __shfl_down_sync(mask, var.operator __half(), delta);
 }