update atari ppo plots

[nuance1979]

• I have marked all applicable categories:
  • exception-raising fix
  • algorithm implementation fix
  • documentation modification
  • new feature
• I have reformatted the code using make format (required)
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Update Atari PPO learning rate to avoid NaN problem.

[codecov-commenter]

Codecov Report

Merging #529 (24a04c0) into master (40289b8) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #529      +/-   ##
==========================================
- Coverage   94.35%   94.33%   -0.03%     
==========================================
  Files          63       63              
  Lines        4251     4251              
==========================================
- Hits         4011     4010       -1     
- Misses        240      241       +1     

Flag	Coverage Δ
unittests	94.33% <ø> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
tianshou/policy/modelfree/npg.py	97.70% <0.00%> (-1.15%)	⬇️

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