[RLlib] New API stack on by default for PPO.

[sven1977]

New API stack on by default for PPO.

To switch back the old API stack, do:

config.api_stack(
    enable_rl_module_and_learner=False,
    enable_env_runner_and_connector_v2=False,
)

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. Dumb question: If PPO is now new API stack by default, can't we just set this in the PPOConfig instead of setting api_stack to True for enable_rl_module_and_learner and enable_env_runner_and_connector_v2?

[simonsays1980]

I thought the hybrid stack is off?