Re-enable device guard

Author: pearu

src/libtorchaudio/iir_cuda.cu

@@ -65,8 +65,7 @@ Tensor cuda_lfilter_core_loop(
 
   STD_TORCH_CHECK(in.size(2) + a_flipped.size(1) - 1 == padded_out.size(2));
 
-  // TODO: enable device guard:
-  // const at::cuda::OptionalCUDAGuard device_guard(in.device());
+  const at::cuda::OptionalCUDAGuard device_guard(in.get_device_index());
 
   const dim3 threads(256);
   const dim3 blocks((N * C + threads.x - 1) / threads.x);