[ROCm][WIP] Improve performance of casted elementwise add operations

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

@@ -231,6 +231,36 @@ static void launch_legacy_kernel(int64_t N, const func_t& f) {
   C10_CUDA_KERNEL_LAUNCH_CHECK();
 }
 
+template <int nt, int vt, typename func_t>
+C10_LAUNCH_BOUNDS_2(nt, 4)
+__global__ void elementwise_kernel_strided(int N, func_t f) {
+  int tid = threadIdx.x;
+  int idx = nt * vt * blockIdx.x + tid;
+  int step = nt * vt * gridDim.x;
+  while (idx < N) {
+#pragma unroll
+    for (int i = 0; i < vt; i++) {
+      if ((idx + nt * i) < N) {
+        f(idx + nt * i);
+      }
+    }
+    idx += step;
+  }
+}
+
+template <int nt, int vt, typename func_t>
+static void launch_legacy_kernel_strided(int64_t N, const func_t& f) {
+  TORCH_INTERNAL_ASSERT(N >= 0 && N <= std::numeric_limits<int32_t>::max());
+  if (N == 0) {
+    return;
+  }
+  dim3 block(nt);
+  dim3 grid(8192);
+  auto stream = at::cuda::getCurrentCUDAStream();
+  elementwise_kernel_strided<nt, vt, func_t><<<grid, block, 0, stream>>>(N, f);
+  C10_CUDA_KERNEL_LAUNCH_CHECK();
+}
+
 template <typename traits, typename func_t, typename index_t, size_t... INDEX>
 C10_HOST_DEVICE typename traits::result_type invoke_impl(
     const func_t& f,
@@ -348,7 +378,7 @@ void gpu_kernel_impl(TensorIteratorBase& iter, const func_t& f) {
       dtypes[i] = iter.dtype(i);
       strides[i] = inner_strides[i];
     }
-    launch_legacy_kernel<512, 1>(numel, [=]GPU_LAMBDA(int idx) {
+    launch_legacy_kernel_strided<512, 4>(numel, [=]GPU_LAMBDA(int idx) {
       void* out = data[0] + strides[0] * idx;
       arg0_t result = invoke(f, &data[1], &strides[1], &dtypes[1], idx);
       c10::cast_and_store<arg0_t>(dtypes[0], out, result);