[ROCm] Set thread_work_size to 16 for vectorized elementwise kernels (#2259)

* thread_work_size of 16 is giving better perf with many workloads

(cherry picked from commit 7edf50c)

Author: jerrymannil

aten/src/ATen/native/cuda/CUDALoops.cuh

@@ -226,8 +226,9 @@ C10_LAUNCH_BOUNDS_1(num_threads())
 __global__ void vectorized_elementwise_kernel(int N, func_t f, array_t data) {
   using traits = function_traits<func_t>;
   constexpr auto io_size = calc_io_size<func_t>();
-#ifdef __gfx942__
-  constexpr int tws = (io_size >= 2) ? 8 : 16;
+#if defined(USE_ROCM) && defined(__gfx942__)
+  // Similar check in launch_vectorized_kernel() as well. Both should be in sync.
+  constexpr int tws = 16;
 #else
   constexpr int tws = elems_per_thread<io_size>();
 #endif
@@ -296,7 +297,7 @@ static inline void launch_vectorized_kernel(
   int vec_size = memory::can_vectorize_up_to<func_t>(data);
   c10::DeviceIndex curDevice = -1;
   AT_CUDA_CHECK(c10::cuda::GetDevice(&curDevice));
-  int tws = at::detail::getCUDAHooks().isGPUArch({"gfx942"}, curDevice) ? ((io_size >= 2) ? 8 : 16) : elems_per_thread<io_size>();
+  int tws = at::detail::getCUDAHooks().isGPUArch({"gfx942"}, curDevice) ? 16 : elems_per_thread<io_size>();
 #else
   using cpp_type = typename function_traits<func_t>::result_type;
   const uint16_t max_vec_size = memory::can_vectorize_up_to<func_t>(data);