Fix pytorch 1.7 imcompatibility issues

[hellock]

No description provided.

[codecov]

Codecov Report

Merging #4103 (6ed04aa) into master (490b6ee) will decrease coverage by 0.02%.
The diff coverage is 65.00%.

@@            Coverage Diff             @@
##           master    #4103      +/-   ##
==========================================
- Coverage   61.91%   61.89%   -0.03%     
==========================================
  Files         224      225       +1     
  Lines       16634    16645      +11     
  Branches     2811     2815       +4     
==========================================
+ Hits        10299    10302       +3     
- Misses       5836     5845       +9     
+ Partials      499      498       -1     

Flag	Coverage Δ
unittests	61.85% <65.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
mmdet/apis/inference.py	20.48% <0.00%> (-1.04%)	⬇️
mmdet/core/anchor/anchor_generator.py	78.98% <ø> (ø)
...mdet/core/bbox/iou_calculators/iou2d_calculator.py	96.77% <ø> (ø)
mmdet/models/dense_heads/atss_head.py	27.94% <ø> (-0.02%)	⬇️
mmdet/models/dense_heads/gfl_head.py	27.35% <ø> (-0.04%)	⬇️
mmdet/models/dense_heads/vfnet_head.py	31.48% <ø> (+0.07%)	⬆️
mmdet/models/losses/focal_loss.py	43.90% <0.00%> (ø)
mmdet/core/utils/dist_utils.py	25.64% <16.66%> (-1.64%)	⬇️
mmdet/models/necks/channel_mapper.py	84.21% <84.21%> (ø)
mmdet/core/utils/__init__.py	100.00% <100.00%> (ø)
... and 6 more

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[dreamflasher]

@ZwwWayne Thanks a lot for this! According to the readme mmdetection does not support 1.7 yet: https://github.com/open-mmlab/mmdetection/blob/master/README.md – is the readme outdated?