fix atss no gt

[jshilong]

Fixed two bug in the ATSS model, each occurred when a single image did not contain any gt boxe and the total batch did not contain any gt boxe respectively

[codecov]

Codecov Report

Merging #3702 into master will decrease coverage by 0.33%.
The diff coverage is 49.39%.

@@            Coverage Diff             @@
##           master    #3702      +/-   ##
==========================================
- Coverage   61.12%   60.78%   -0.34%     
==========================================
  Files         216      217       +1     
  Lines       15315    15377      +62     
  Branches     2607     2629      +22     
==========================================
- Hits         9361     9347      -14     
- Misses       5489     5561      +72     
- Partials      465      469       +4     

Flag	Coverage Δ
#unittests	60.78% <49.39%> (-0.34%)	⬇️

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

Impacted Files	Coverage Δ
mmdet/apis/test.py	12.84% <0.00%> (-0.25%)	⬇️
mmdet/core/post_processing/merge_augs.py	80.35% <ø> (ø)
mmdet/datasets/lvis.py	13.24% <0.00%> (-1.15%)	⬇️
mmdet/models/detectors/base.py	50.76% <ø> (ø)
mmdet/models/dense_heads/atss_head.py	28.13% <25.00%> (+0.06%)	⬆️
mmdet/core/bbox/samplers/ohem_sampler.py	83.78% <50.00%> (-2.33%)	⬇️
mmdet/datasets/coco.py	45.63% <50.00%> (+0.24%)	⬆️
mmdet/models/backbones/res2net.py	82.67% <52.00%> (-7.62%)	⬇️
mmdet/core/anchor/anchor_generator.py	77.53% <100.00%> (ø)
mmdet/datasets/__init__.py	100.00% <100.00%> (ø)
... and 11 more

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[hellock]

ping @jshilong