add comments about origin of reference implementation (#20763)

src/plugins/intel_gpu/src/kernel_selector/cl_kernels/non_max_suppression_gpu_ref.cl

@@ -1,6 +1,9 @@
 // Copyright (C) 2018-2023 Intel Corporation
 // SPDX-License-Identifier: Apache-2.0
 //
+// Copyright (c) Facebook, Inc. and its affiliates.
+// The implementation for rotated boxes intersection is based on the code from:
+// https://github.com/facebookresearch/detectron2/blob/v0.6/detectron2/layers/csrc/box_iou_rotated/box_iou_rotated_utils.h
 
 #include "include/batch_headers/fetch_data.cl"