[ROCm] Unroll loads in global_reduce (#2554)

cherry-pick of pytorch#161181

Fixes SWDEV-545710

Author: jerrymannil

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

@@ -831,6 +831,23 @@ struct ReduceOp {
       } else {
         index_t input_offset = threadIdx.y;
         index_t step = blockDim.y;
+#ifdef USE_ROCM // Prefetch loads to better hide their latency
+        #define PRFCH 4
+        for (; input_offset < config.ctas_per_output; input_offset += step*PRFCH) {
+         arg_vec_t next[PRFCH];
+         #pragma unroll
+         for (int u = 0; (u < PRFCH) && (input_offset + u*step < config.ctas_per_output); u++) {
+          index_t idx = config.staging_memory_offset(input_offset + u*step);
+          next[u] = reduce_buffer[idx];
+         }
+         for (int u = 0; (u < PRFCH) && (input_offset + u*step < config.ctas_per_output); u++) {
+          #pragma unroll
+          for (int i = 0; i < output_vec_size; i++) {
+            value[i] = ops.combine(value[i], next[u][i]);
+          }
+         }
+        }
+#else
         for (; input_offset < config.ctas_per_output; input_offset += step) {
           index_t idx = config.staging_memory_offset(input_offset);
           arg_vec_t next = reduce_buffer[idx];
@@ -839,6 +856,7 @@ struct ReduceOp {
             value[i] = ops.combine(value[i], next[i]);
           }
         }
+#endif
       }
       value = block_y_reduce<output_vec_size>(value, shared_memory);
       if (config.should_block_x_reduce()) {