add Atari SAC examples

[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)
• I have checked the code using make commit-checks (required)
• If applicable, I have mentioned the relevant/related issue(s)
• If applicable, I have listed every items in this Pull Request below

• Add Atari (discrete) SAC examples;
• Fix a bug in Discrete SAC evaluation; default to deterministic mode.

[codecov-commenter]

Codecov Report

Merging #657 (de7dfa9) into master (5ecea24) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #657   +/-   ##
=======================================
  Coverage   93.62%   93.63%           
=======================================
  Files          71       71           
  Lines        4755     4757    +2     
=======================================
+ Hits         4452     4454    +2     
  Misses        303      303           

Flag	Coverage Δ
unittests	93.63% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
tianshou/policy/modelfree/discrete_sac.py	86.56% <100.00%> (+0.41%)	⬆️

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[Trinkle23897]

Fix an off-by-one bug in trainer iterator;

It is a serious bug so I stole it to #659.

[Trinkle23897]

ping me if it's ready

[nuance1979]

ping me if it's ready

It's ready.

I tried some parameter-tuning but couldn't improve the Breakout result. The original paper also reported very low score for Breakout game. I see no point going further. Please review. @Trinkle23897