enhance nms for mask rcnn

paddle/fluid/operators/detection/bbox_util.h

@@ -93,5 +93,25 @@ void BboxOverlaps(const framework::Tensor& r_boxes,
   }
 }
 
+template <class T>
+void SliceOneClass(const platform::DeviceContext& ctx,
+                   const framework::Tensor& items, const int class_id,
+                   framework::Tensor* one_class_item) {
+  T* item_data = one_class_item->mutable_data<T>(ctx.GetPlace());
+  const T* items_data = items.data<T>();
+  const int64_t num_item = items.dims()[0];
+  const int class_num = items.dims()[1];
+  int item_size = 1;
+  if (items.dims().size() == 3) {
+    item_size = items.dims()[2];
+  }
+  for (int i = 0; i < num_item; ++i) {
+    for (int j = 0; j < item_size; ++j) {
+      item_data[i * item_size + j] =
+          items_data[i * class_num * item_size + class_id * item_size + j];
+    }
+  }
+}
+
 }  // namespace operators
 }  // namespace paddle