Revise function of is_init flag in BaseModule

[MeowZheng]

Hi,

In this pr, I change the is_init flag function. When it is True means that init_weight has been called.

Thank you @jshilong for your kind suggestion.

[codecov]

Codecov Report

Merging #898 (292cdc5) into master (73bff4e) will increase coverage by 0.00%.
The diff coverage is 73.33%.

❗ Current head 292cdc5 differs from pull request most recent head ad1b264. Consider uploading reports for the commit ad1b264 to get more accurate results

@@           Coverage Diff           @@
##           master     #898   +/-   ##
=======================================
  Coverage   66.58%   66.58%           
=======================================
  Files         145      145           
  Lines        8828     8841   +13     
  Branches     1605     1606    +1     
=======================================
+ Hits         5878     5887    +9     
- Misses       2633     2637    +4     
  Partials      317      317           

Flag	Coverage Δ
unittests	66.58% <73.33%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
mmcv/runner/base_module.py	79.41% <71.42%> (-6.31%)	⬇️
mmcv/runner/__init__.py	100.00% <100.00%> (ø)

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