[Fix]: add header file for std::iota(#803) (#820)

* add header to std::iota

* fix lint

* fix lint for cpp

mmcv/ops/csrc/onnxruntime/cpu/nms.cpp

@@ -6,6 +6,7 @@
 #include <cmath>
 #include <iostream>
 #include <iterator>
+#include <numeric>  // std::iota
 #include <vector>
 
 #include "../ort_mmcv_utils.h"

mmcv/ops/nms.py

@@ -291,7 +291,8 @@ def batched_nms(boxes, scores, idxs, nms_cfg, class_agnostic=False):
     nms_op = eval(nms_type)
 
     split_thr = nms_cfg_.pop('split_thr', 10000)
-    if boxes_for_nms.shape[0] < split_thr:
+    # Won't split to multiple nms nodes when exporting to onnx
+    if boxes_for_nms.shape[0] < split_thr or torch.onnx.is_in_onnx_export():
         dets, keep = nms_op(boxes_for_nms, scores, **nms_cfg_)
         boxes = boxes[keep]
         scores = dets[:, -1]