Support Actor preprocessing network reuse for continuous case, fixes in DQN network

[opcode81]

• I have added the correct label(s) to this Pull Request or linked the relevant issue(s)
• I have provided a description of the changes in this Pull Request
• I have added documentation for my changes
• If applicable, I have added tests to cover my changes.
• I have reformatted the code using poe format
• I have checked style and types with poe lint and poe type-check
• (Optional) I ran tests locally with poe test
  (or a subset of them with poe test-reduced) ,and they pass
• (Optional) I have tested that documentation builds correctly with poe doc-build

This PR fixes a bug in DQN and lifts a limination in reusing the actor's preprocessing network for continuous environments.

• atari_network.DQN:
  • Fix input validation
  • Fix output_dim not being set if features_only=True and output_dim_added_layer not None
• continuous.Critic:
  • Add flag apply_preprocess_net_to_obs_only to allow the
    preprocessing network to be applied to the observations only (without
    the actions concatenated), which is essential for the case where we want
    to reuse the actor's preprocessing network
  • CriticFactoryReuseActor: Use the flag, fixing the case where we want to reuse an actor's
    preprocessing network for the critic (must be applied before concatenating
    the actions)
• Minor improvements in docs/docstrings

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 43.10345% with 33 lines in your changes are missing coverage. Please review.

Project coverage is 86.07%. Comparing base (081aded) to head (40f7724).

Files	Patch %	Lines
tianshou/utils/pickle.py	31.11%	31 Missing ⚠️
tianshou/highlevel/module/critic.py	0.00%	1 Missing ⚠️
tianshou/utils/net/continuous.py	90.00%	1 Missing ⚠️

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1128      +/-   ##
==========================================
- Coverage   86.36%   86.07%   -0.29%     
==========================================
  Files         102      103       +1     
  Lines        8579     8632      +53     
==========================================
+ Hits         7409     7430      +21     
- Misses       1170     1202      +32     

Flag	Coverage Δ
unittests	86.07% <43.10%> (-0.29%)	⬇️

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.