[RLlib] RLModule: Simplify defining custom distribution classes and add better defaults.

[sven1977]

RLModule: Simplify defining custom distribution classes and add better defaults.

• In most cases, users won't have to override their custom RLModule's get_inference|exploration|train_action_dist_cls methods anymore.
• Users can still override the above three methods to customize their distribution classes.
• OR: Users can still use custom Catalogs to define custom distributions (however, we'll probably deprecate this class at some point and replace it with simpler utility methods to build standard model components).

Why are these changes needed?

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[simonsays1980]

LGTM. Only one question about composite action spaces.

[simonsays1980]

Same here :)