[Feature]: support dynamic shape for grid_anchor

[RunningLeon]

This PR modified AnchorHead to support dynamic shape for grid_anchor when exporting to ONNX.

[codecov]

Codecov Report

Merging #4684 (e126c53) into master (e1599e7) will decrease coverage by 0.53%.
The diff coverage is 48.39%.

@@            Coverage Diff             @@
##           master    #4684      +/-   ##
==========================================
- Coverage   64.15%   63.62%   -0.54%     
==========================================
  Files         251      252       +1     
  Lines       19390    19569     +179     
  Branches     3265     3329      +64     
==========================================
+ Hits        12440    12451      +11     
- Misses       6299     6441     +142     
- Partials      651      677      +26     

Flag	Coverage Δ
unittests	63.62% <48.39%> (-0.50%)	⬇️

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Impacted Files	Coverage Δ
mmdet/core/export/pytorch2onnx.py	17.02% <0.00%> (-0.38%)	⬇️
mmdet/datasets/builder.py	65.27% <0.00%> (ø)
mmdet/datasets/samplers/group_sampler.py	18.75% <0.00%> (-0.24%)	⬇️
mmdet/models/dense_heads/fcos_head.py	58.96% <0.00%> (-2.02%)	⬇️
mmdet/models/dense_heads/yolo_head.py	32.29% <0.00%> (-1.76%)	⬇️
mmdet/models/dense_heads/ga_rpn_head.py	27.27% <15.00%> (-1.50%)	⬇️
mmdet/models/dense_heads/corner_head.py	73.72% <20.00%> (-0.73%)	⬇️
mmdet/datasets/samplers/distributed_sampler.py	33.33% <25.00%> (ø)
...heads/roi_extractors/single_level_roi_extractor.py	69.38% <28.57%> (-0.83%)	⬇️
mmdet/core/post_processing/merge_augs.py	70.00% <37.50%> (-10.36%)	⬇️
... and 53 more

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