Add argument features_extractor to ActorCriticPolicy.extract_features

[qgallouedec]

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[qgallouedec]

To fix mypy:

File "/home/runner/work/stable-baselines3/stable-baselines3/stable_baselines3/common/policies.py", line 645, in ActorCriticPolicy: Overriding method signature mismatch [signature-mismatch]
  Base signature: 'def BaseModel.extract_features(self, obs: Any, features_extractor: stable_baselines3.common.torch_layers.BaseFeaturesExtractor) -> Any'.
  Subclass signature: 'def ActorCriticPolicy.extract_features(self, obs: Any) -> Union[Any, Tuple[Any, Any]]'.
  Not enough positional parameters in overriding method.

[araffin]

LGTM, thanks =)
similar fix is needed for sb3 contrib?