[Feature] Add Kernel head

[FANG-MING]

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Motivation

Add Kernel head which is a rpn head used in K-net.

Modification

Add Kernel head

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

Merging #7638 (3e26931) into dev (6b87ac2) will increase coverage by 0.68%. The diff coverage is 81.75%.

:exclamation: Current head 3e26931 differs from pull request most recent head 3bccc7d. Consider uploading reports for the commit 3bccc7d to get more accurate results

@@            Coverage Diff             @@
##              dev    #7638      +/-   ##
==========================================
+ Coverage   63.41%   64.10%   +0.68%     
==========================================
  Files         340      346       +6     
  Lines       27493    28117     +624     
  Branches     4645     4744      +99     
==========================================
+ Hits        17436    18024     +588     
- Misses       9131     9150      +19     
- Partials      926      943      +17     

Flag	Coverage Δ
unittests	64.07% <81.75%> (+0.68%)	:arrow_up:

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

Impacted Files	Coverage Δ
mmdet/apis/test.py	12.50% <0.00%> (-0.90%)	:arrow_down:
mmdet/core/bbox/assigners/max_iou_assigner.py	73.68% <ø> (+1.31%)	:arrow_up:
mmdet/core/post_processing/bbox_nms.py	79.10% <ø> (ø)
mmdet/core/utils/__init__.py	100.00% <ø> (ø)
mmdet/datasets/pipelines/instaboost.py	15.00% <0.00%> (ø)
...mdet/models/dense_heads/free_anchor_retina_head.py	16.00% <0.00%> (-0.22%)	:arrow_down:
mmdet/models/dense_heads/ssd_head.py	85.84% <ø> (ø)
mmdet/models/necks/fpn.py	94.66% <ø> (ø)
mmdet/core/utils/dist_utils.py	23.59% <15.38%> (-1.41%)	:arrow_down:
mmdet/utils/misc.py	64.10% <22.22%> (-35.90%)	:arrow_down:
... and 46 more

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[ZwwWayne]

Please clean unnecessary files and resolve the conflict.