Clean up handling of output_dim

[opcode81]

• I have marked all applicable categories:
  • exception-raising bug
  • RL algorithm bug
  • documentation request (i.e. "X is missing from the documentation.")
  • new feature request
  • design request (i.e. "X should be changed to Y.")
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• I have mentioned version numbers, operating system and environment, where applicable:

  import tianshou, gymnasium as gym, torch, numpy, sys
  print(tianshou.__version__, gym.__version__, torch.__version__, numpy.__version__, sys.version, sys.platform)

There are contrived mechanisms for the retrieval of the "output_dim" attribute of nn.Modules in several places (see usages of newly introduced function get_output_dim; there may be others). The clean way of handling this is to change the respective interfaces to a more specfic class that explicitly supports the retrieval of the output dimension. (The high-level API has already partly addressed this by adding corresponding methods to base classes.)