Fix self.rpn_head in rpn head

[ZwwWayne]

Fix #3326

[codecov]

Codecov Report

Merging #3808 into master will decrease coverage by 0.00%.
The diff coverage is 64.80%.

@@            Coverage Diff             @@
##           master    #3808      +/-   ##
==========================================
- Coverage   61.12%   61.11%   -0.01%     
==========================================
  Files         216      213       -3     
  Lines       15315    15213     -102     
  Branches     2607     2587      -20     
==========================================
- Hits         9361     9298      -63     
+ Misses       5489     5446      -43     
- Partials      465      469       +4     

Flag	Coverage Δ
#unittests	61.11% <64.80%> (-0.01%)	⬇️

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/__init__.py	100.00% <ø> (ø)
mmdet/core/post_processing/merge_augs.py	80.35% <ø> (ø)
mmdet/datasets/lvis.py	13.24% <0.00%> (-1.15%)	⬇️
mmdet/models/dense_heads/rpn_test_mixin.py	83.87% <0.00%> (ø)
mmdet/models/dense_heads/atss_head.py	28.44% <40.00%> (+0.37%)	⬆️
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/utils/collect_env.py	40.00% <66.66%> (+17.08%)	⬆️
... and 33 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b4e2155...91c541f. Read the comment docs.