Support batch_size=None and use it in various scripts

[MischaPanch]

Closes #986

[Trinkle23897]

My bad, lgtm

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (f134bc2) 88.11% compared to head (7c0fb33) 88.09%.

Files	Patch %	Lines
tianshou/data/buffer/base.py	75.00%	1 Missing ⚠️
tianshou/data/buffer/manager.py	80.00%	1 Missing ⚠️
tianshou/trainer/base.py	83.33%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #993      +/-   ##
==========================================
- Coverage   88.11%   88.09%   -0.02%     
==========================================
  Files          96       96              
  Lines        7514     7527      +13     
==========================================
+ Hits         6621     6631      +10     
- Misses        893      896       +3     

Flag	Coverage Δ
unittests	88.09% <89.28%> (-0.02%)	⬇️

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