[RLlib] No Preprocessors; preparatory PR #1

[sven1977]

This PR was motivated in preparation for soon allowing individual observation components to be addressed by the trajectory view API, for example to enable frame-stacking for individual observation components within a complex observation space (Tuple|Dict). Also, soon soft-deprecating RLlib's Preprocessor API should increase transparency for the users and allow batched, model-based preprocessing of observations. Observations will arrive in the model exactly as they are returned by the env.

This PR is a preparatory PR that lays some groundwork in order to fully support this feature (in 1 follow up PR).

• New config setting for preprocessor_pref: None; Set to None for disabling Preprocessors altogether (not even use NoPreprocessor class anymore).
• ComplexInputModel (which already exists and handles complex, unflattened inputs) was enhanced to support Dict inputs as well (so far, only Tuple inputs).
• Some docs strings and comments added.

Why are these changes needed?

Related issue number

Checks

• 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 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 :(