[BUG]fix init of convfc head

[jshilong]

Motivation

mmdetection/mmdet/models/roi_heads/bbox_heads/convfc_bbox_head.py

Line 103 in c88509c

layer='Linear',

This line will override initializers for fc_cls and fc_reg because they are also nn.Linear.
which is not consistent with the old way to initialize fc_cls and fc_reg before #4750.

Closes #6238

BC breaking

None

[codecov]

Codecov Report

Merging #6279 (a9d5b62) into master (0fcea45) will decrease coverage by 6.40%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #6279      +/-   ##
==========================================
- Coverage   68.27%   61.86%   -6.41%     
==========================================
  Files         304      315      +11     
  Lines       23842    25163    +1321     
  Branches     3939     4185     +246     
==========================================
- Hits        16278    15567     -711     
- Misses       6721     8782    +2061     
+ Partials      843      814      -29     

Flag	Coverage Δ
unittests	61.84% <0.00%> (-6.44%)	⬇️

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

Impacted Files	Coverage Δ
mmdet/core/export/model_wrappers.py	0.00% <0.00%> (ø)
...et/models/roi_heads/bbox_heads/convfc_bbox_head.py	55.88% <ø> (-10.79%)	⬇️
mmdet/models/roi_heads/mask_heads/grid_head.py	7.69% <0.00%> (-87.70%)	⬇️
mmdet/models/roi_heads/mask_scoring_roi_head.py	16.98% <0.00%> (-75.48%)	⬇️
mmdet/models/dense_heads/nasfcos_head.py	28.12% <0.00%> (-68.75%)	⬇️
mmdet/models/seg_heads/panoptic_fpn_head.py	27.65% <0.00%> (-65.82%)	⬇️
mmdet/models/roi_heads/grid_roi_head.py	13.04% <0.00%> (-57.61%)	⬇️
mmdet/models/necks/nas_fpn.py	14.28% <0.00%> (-55.56%)	⬇️
...t/models/detectors/panoptic_two_stage_segmentor.py	16.66% <0.00%> (-52.09%)	⬇️
mmdet/models/utils/normed_predictor.py	33.33% <0.00%> (-50.01%)	⬇️
... and 135 more

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[ZwwWayne]

I only see comments added, does this PR fix anything?

[jshilong]

I only see comments added, does this PR fix anything?

I remove layer='Linear', from init_cfg which would overwide all linear layers when doing the initialization