[Enhance] Modify the Fcos3d init_weights methods

[xiliu8006]

Motivation

Replace the init_weights() with init_cfg.

Modification

Modified the Fcos3d init_cfg

[codecov]

Codecov Report

Merging #705 (3e586eb) into master (1f79fc7) will increase coverage by 0.07%.
The diff coverage is 50.00%.

❗ Current head 3e586eb differs from pull request most recent head 84b271c. Consider uploading reports for the commit 84b271c to get more accurate results

@@            Coverage Diff             @@
##           master     #705      +/-   ##
==========================================
+ Coverage   48.93%   49.00%   +0.07%     
==========================================
  Files         208      208              
  Lines       15861    15839      -22     
  Branches     2543     2529      -14     
==========================================
+ Hits         7761     7762       +1     
+ Misses       7606     7581      -25     
- Partials      494      496       +2     

Flag	Coverage Δ
unittests	49.00% <50.00%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
...et3d/models/dense_heads/anchor_free_mono3d_head.py	65.31% <33.33%> (+6.67%)	⬆️
mmdet3d/models/dense_heads/fcos_mono3d_head.py	12.10% <66.66%> (+0.13%)	⬆️

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