Customize runner type

[yhcao6]

Add support to customize runner type ('EpochBasedRunner', 'IterBasedRunner') and merge the 'total_epoch' attribute in the config with the newly added runner attribute as follows:

runner = dict(type='EpochBasedRunner', max_epochs=12)

[codecov]

Codecov Report

Merging #4570 (cacb1de) into master (c2afc5a) will decrease coverage by 0.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #4570      +/-   ##
==========================================
- Coverage   64.31%   64.29%   -0.02%     
==========================================
  Files         250      250              
  Lines       19249    19286      +37     
  Branches     3252     3264      +12     
==========================================
+ Hits        12380    12400      +20     
- Misses       6223     6234      +11     
- Partials      646      652       +6     

Flag	Coverage Δ
unittests	64.25% <33.33%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
mmdet/models/dense_heads/gfl_head.py	27.35% <0.00%> (ø)
mmdet/apis/train.py	16.21% <9.09%> (-0.46%)	⬇️
mmdet/datasets/pipelines/transforms.py	72.60% <33.33%> (-0.31%)	⬇️
mmdet/core/evaluation/eval_hooks.py	51.70% <40.47%> (-3.14%)	⬇️
mmdet/models/dense_heads/corner_head.py	76.35% <0.00%> (+1.90%)	⬆️
mmdet/models/detectors/cornernet.py	100.00% <0.00%> (+5.12%)	⬆️

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[ZwwWayne]

LGTM after resolving comments. Please @jshilong also have a look.

[ZwwWayne]

May also enrich the PR message to describe the supported feature and migration code in detail. Thus other users could understand the PR more clearly as this modification is important.