Moved subbuffer-related functionality from Collector to Buffer

[MischaPanch]

Internal improvements post #1196

@maxhuettenrauch FYI

@Trinkle23897 I'm going to merge this directly after CI runs through as it's small internal improvements in new functionality, which are well tested and documented inside the codebase, no need for review IMO.

Overall improvements in documentation will follow soon

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 86.95652% with 6 lines in your changes missing coverage. Please review.

Project coverage is 85.28%. Comparing base (7640c6e) to head (c903f72).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
tianshou/data/buffer/base.py	85.36%	6 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1214      +/-   ##
==========================================
- Coverage   85.30%   85.28%   -0.03%     
==========================================
  Files         102      102              
  Lines        9074     9078       +4     
==========================================
+ Hits         7741     7742       +1     
- Misses       1333     1336       +3     

Flag	Coverage Δ
unittests	85.28% <86.95%> (-0.03%)	⬇️

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