Fix #3950, avoid zero or too small value for beta in dynamic rcnn

[v-qjqs]

This PR fix #3950, avoid zero or too small value for beta in dynamic rcnn

[codecov]

Codecov Report

Merging #4303 (600d419) into master (43b1909) will decrease coverage by 0.06%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master    #4303      +/-   ##
==========================================
- Coverage   62.70%   62.63%   -0.07%     
==========================================
  Files         232      232              
  Lines       17607    17609       +2     
  Branches     2945     2946       +1     
==========================================
- Hits        11041    11030      -11     
- Misses       6027     6042      +15     
+ Partials      539      537       -2     

Flag	Coverage Δ
unittests	62.59% <25.00%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
mmdet/models/roi_heads/dynamic_roi_head.py	24.24% <25.00%> (-0.76%)	⬇️
mmdet/models/roi_heads/test_mixins.py	45.73% <0.00%> (-8.53%)	⬇️

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