[ROCm] Improve perf for elementwise broadcast with mixed dtype (#2674)

* cherry-pick of pytorch/pytorch@2aadcea

Author: jerrymannil

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

@@ -999,12 +999,41 @@ void gpu_kernel_impl(TensorIteratorBase& iter, const func_t& f) {
       dtypes[i] = iter.dtype(i);
     }
     auto offset_calc = ::make_offset_calculator<traits::arity + 1>(iter);
+#ifdef USE_ROCM
+    constexpr int grp_sz = 128;
+    launch_legacy_kernel_manual_unroll<grp_sz, 4>(numel, [=] GPU_LAMBDA(int idx, bool unrl) {
+      if (unrl) {
+        auto offsets0 = offset_calc.get(idx);
+        auto offsets1 = offset_calc.get(idx + grp_sz);
+        auto offsets2 = offset_calc.get(idx + grp_sz * 2);
+        auto offsets3 = offset_calc.get(idx + grp_sz * 3);
+        void* out0 = data[0] + offsets0[0];
+        void* out1 = data[0] + offsets1[0];
+        void* out2 = data[0] + offsets2[0];
+        void* out3 = data[0] + offsets3[0];
+        arg0_t result0 = invoke(f, &data[1], &offsets0[1], &dtypes[1], 1);
+        arg0_t result1 = invoke(f, &data[1], &offsets1[1], &dtypes[1], 1);
+        arg0_t result2 = invoke(f, &data[1], &offsets2[1], &dtypes[1], 1);
+        arg0_t result3 = invoke(f, &data[1], &offsets3[1], &dtypes[1], 1);
+        c10::cast_and_store<arg0_t>(dtypes[0], out0, result0);
+        c10::cast_and_store<arg0_t>(dtypes[0], out1, result1);
+        c10::cast_and_store<arg0_t>(dtypes[0], out2, result2);
+        c10::cast_and_store<arg0_t>(dtypes[0], out3, result3);
+      } else {
+        auto offsets = offset_calc.get(idx);
+        void* out = data[0] + offsets[0];
+        arg0_t result = invoke(f, &data[1], &offsets[1], &dtypes[1], 1);
+        c10::cast_and_store<arg0_t>(dtypes[0], out, result);
+      }
+    });
+#else
     launch_legacy_kernel<128, 4>(numel, [=] GPU_LAMBDA(int idx) {
       auto offsets = offset_calc.get(idx);
       void* out = data[0] + offsets[0];
       arg0_t result = invoke(f, &data[1], &offsets[1], &dtypes[1], 1);
       c10::cast_and_store<arg0_t>(dtypes[0], out, result);
     });
+#endif
   }
 }