optimize vdn algorithm v3

[strivebfq]

Description

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have ensured make test pass (required).
• I have checked the code using make commit-checks (required).

[codecov]

Codecov Report

Merging #108 (53c9276) into main (0785c7a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   45.22%   45.24%   +0.01%     
==========================================
  Files         147      147              
  Lines        9169     9171       +2     
==========================================
+ Hits         4147     4149       +2     
  Misses       5022     5022              

Flag	Coverage Δ
unittests	45.24% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
openrl/runners/common/vdn_agent.py	56.66% <ø> (ø)
openrl/algorithms/vdn.py	64.81% <100.00%> (+0.66%)	⬆️

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