Support for using random agents, improvements in CollectStats

[MischaPanch]

Random action agents can be useful for tests, but also as a baseline.

Also (independent change):

• improved CollectStats for action_std
• Batch: extension for changing shape to atleast_2d, including "casting" torch distributions to atleast_2d

One thing is a bit weird in this implementation: in high level interfaces the optim_factory is obligatory. For random actions it doesn't make sense, of course, but making it optional would require changes and additional type checks in several places. @opcode81 what do you think about this - should we make it optional?

@Trinkle23897 a quick glance should be enough, nothing controversial. @opcode81 is on vacation and won't have time to review

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

Attention: Patch coverage is 92.17391% with 9 lines in your changes missing coverage. Please review.

Project coverage is 85.30%. Comparing base (002ffd9) to head (4e03191).

Files	Patch %	Lines
tianshou/utils/net/common.py	90.62%	3 Missing ⚠️
tianshou/data/batch.py	89.47%	2 Missing ⚠️
tianshou/policy/base.py	83.33%	2 Missing ⚠️
tianshou/highlevel/agent.py	75.00%	1 Missing ⚠️
tianshou/highlevel/experiment.py	80.00%	1 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1207      +/-   ##
==========================================
+ Coverage   85.25%   85.30%   +0.05%     
==========================================
  Files         102      102              
  Lines        8982     9074      +92     
==========================================
+ Hits         7658     7741      +83     
- Misses       1324     1333       +9     

Flag	Coverage Δ
unittests	85.30% <92.17%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[MischaPanch]

@maxhuettenrauch I improved the collect-stats for action dists in 6f8648a and b0ba423, also adding tests for stats collection. FYI

[MischaPanch]

Since it's mostly an extension (apart from a minor renaming of an existing class), I'm going to merge this. Feel free to comment post-merge if there's anything to be discussed