[doc]: add doc for pytorch2onnx

[RunningLeon]

Hi,
Add a document for the usage and the list of supported models of pytorch2onnx .

[codecov]

Codecov Report

Merging #4271 (0e7d985) into master (bec3fc1) will decrease coverage by 0.04%.
The diff coverage is 62.94%.

@@            Coverage Diff             @@
##           master    #4271      +/-   ##
==========================================
- Coverage   62.69%   62.65%   -0.05%     
==========================================
  Files         232      235       +3     
  Lines       17603    17821     +218     
  Branches     2945     2984      +39     
==========================================
+ Hits        11037    11165     +128     
- Misses       6027     6112      +85     
- Partials      539      544       +5     

Flag	Coverage Δ
unittests	62.61% <62.94%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
...mdet/core/bbox/iou_calculators/iou2d_calculator.py	96.77% <ø> (ø)
mmdet/datasets/pipelines/formating.py	66.37% <ø> (ø)
mmdet/models/backbones/resnet.py	99.30% <ø> (ø)
mmdet/models/dense_heads/yolo_head.py	34.04% <ø> (ø)
mmdet/models/roi_heads/trident_roi_head.py	23.63% <23.63%> (ø)
mmdet/models/roi_heads/dynamic_roi_head.py	25.37% <25.00%> (+0.37%)	⬆️
mmdet/models/detectors/trident_faster_rcnn.py	43.33% <43.33%> (ø)
mmdet/datasets/samplers/distributed_sampler.py	33.33% <50.00%> (+3.92%)	⬆️
mmdet/models/backbones/trident_resnet.py	83.73% <83.73%> (ø)
mmdet/models/backbones/__init__.py	100.00% <100.00%> (ø)
... and 10 more

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[hellock]

The tutorial is not added to the doc index.

[RunningLeon]

@hellock
This doc is added as reference in MMDetection model to ONNX (experimental) part in docs/usefull_tools.md.