[Refactoring] Revise function of layers and override keys in init_cfg

[MeowZheng]

Hi,

As developers would like to only initialize parameters with attribute names, I just revise the initialization mechanism. If we don't define layer key or override key, it will not initialize anything; if we define override but don't define layer, it will initialize parameters with the attribute name in override; if we only define layer, it just initialize the layer in layer key; if we define override and layer, override has higher priority and will override initialization mechanism.

Moreover, for ease of use, if we only define override, and override without type key, the type key and other kwargs will merge in override.

[codecov]

Codecov Report

Merging #893 (877c152) into master (73bff4e) will decrease coverage by 0.12%.
The diff coverage is 80.00%.

❗ Current head 877c152 differs from pull request most recent head d4eef56. Consider uploading reports for the commit d4eef56 to get more accurate results

@@            Coverage Diff             @@
##           master     #893      +/-   ##
==========================================
- Coverage   66.58%   66.45%   -0.13%     
==========================================
  Files         145      145              
  Lines        8828     8845      +17     
  Branches     1605     1603       -2     
==========================================
  Hits         5878     5878              
- Misses       2633     2641       +8     
- Partials      317      326       +9     

Flag	Coverage Δ
unittests	66.45% <80.00%> (-0.13%)	⬇️

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Impacted Files	Coverage Δ
mmcv/runner/base_module.py	79.41% <71.42%> (-6.31%)	⬇️
mmcv/cnn/utils/weight_init.py	91.27% <84.00%> (-7.54%)	⬇️
mmcv/runner/__init__.py	100.00% <100.00%> (ø)

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