[Enhance]: Better support yolov3 pytorch2onnx

[RunningLeon]

Better support yolov3 pytorch2onnx with TensorRT as backend. PR(#3956)

[codecov]

Codecov Report

Merging #4083 (51b9cd5) into master (5d07dda) will decrease coverage by 0.21%.
The diff coverage is 42.67%.

@@            Coverage Diff             @@
##           master    #4083      +/-   ##
==========================================
- Coverage   62.17%   61.96%   -0.22%     
==========================================
  Files         221      225       +4     
  Lines       16314    16642     +328     
  Branches     2760     2815      +55     
==========================================
+ Hits        10144    10313     +169     
- Misses       5678     5830     +152     
- Partials      492      499       +7     

Flag	Coverage Δ
unittests	61.92% <42.67%> (-0.22%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmdet/core/anchor/anchor_generator.py	78.98% <ø> (ø)
mmdet/core/utils/misc.py	100.00% <ø> (+33.33%)	⬆️
mmdet/models/dense_heads/atss_head.py	27.94% <0.00%> (-0.26%)	⬇️
mmdet/models/dense_heads/gfl_head.py	27.35% <ø> (-0.04%)	⬇️
mmdet/models/dense_heads/rpn_head.py	84.93% <0.00%> (ø)
mmdet/models/dense_heads/yolo_head.py	34.04% <0.00%> (-0.19%)	⬇️
mmdet/core/utils/dist_utils.py	25.64% <16.66%> (-1.64%)	⬇️
mmdet/apis/train.py	16.66% <25.00%> (-0.80%)	⬇️
mmdet/models/dense_heads/vfnet_head.py	31.48% <31.48%> (ø)
mmdet/apis/inference.py	21.51% <50.00%> (+0.78%)	⬆️
... and 22 more

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