[Fix]: Fix initialization bug of override cfg

[ZwwWayne]

This PR fixes the bug in the initialize function, which will change the original init_cfg after override some keys.
This behavior will cause an error when one config is used in many other modules as the config will be changed after it has been used once.
One test case that will cause the failure is here.

[MeowZheng]

Many thanks for fixing this bug.

[codecov]

Codecov Report

Merging #918 (9488839) into master (9d80f56) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 9488839 differs from pull request most recent head 024d5f4. Consider uploading reports for the commit 024d5f4 to get more accurate results

@@           Coverage Diff           @@
##           master     #918   +/-   ##
=======================================
  Coverage   64.86%   64.87%           
=======================================
  Files         148      148           
  Lines        9193     9195    +2     
  Branches     1647     1647           
=======================================
+ Hits         5963     5965    +2     
  Misses       2903     2903           
  Partials      327      327           

Flag	Coverage Δ
unittests	64.87% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
mmcv/cnn/utils/weight_init.py	91.66% <100.00%> (+0.09%)	⬆️

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