Question about multiple FlattenExtractor components in ActorCriticPolicy

Hi SB3 team,

Thank you for your great work on this library!

I have a question regarding the ActorCriticPolicy architecture . I noticed that there are three separate FlattenExtractor instances: features_extractor, pi_features_extractor, and vf_features_extractor.

```
ActorCriticPolicy(
(features_extractor): FlattenExtractor(
(flatten): Flatten(start_dim=1, end_dim=-1)
)
(pi_features_extractor): FlattenExtractor(
(flatten): Flatten(start_dim=1, end_dim=-1)
)
(vf_features_extractor): FlattenExtractor(
(flatten): Flatten(start_dim=1, end_dim=-1)
)
(mlp_extractor): MlpExtractor(
(policy_net): Sequential(
(0): Linear(in_features=4, out_features=64, bias=True)
(1): Tanh()
(2): Linear(in_features=64, out_features=64, bias=True)
(3): Tanh()
)
(value_net): Sequential(
(0): Linear(in_features=4, out_features=64, bias=True)
(1): Tanh()
(2): Linear(in_features=64, out_features=64, bias=True)
(3): Tanh()
)
)
(action_net): Linear(in_features=64, out_features=2, bias=True)
(value_net): Linear(in_features=64, out_features=1, bias=True)
)
```

Could you please clarify what the purpose of each of these is? Specifically:

Why are there three flatten extractors instead of just one?

What is the difference between features_extractor, pi_features_extractor, and vf_features_extractor in this context?

My guess is that features_extractor is used for shared feature extraction, while pi_features_extractor and vf_features_extractor are used for separate, dedicated extraction paths for the policy and value networks, respectively. I also assume that these cannot be active at the same time — that is, when the shared extractor is used, the other two are inactive, and vice versa.

Is this understanding correct?

As a suggestion: it would be helpful if the documentation included a few concrete examples showing the actual output structure of the networks. That would make it easier to understand how the components interact.

Thanks again!

Start by reading the ActorCriticPolicy implementation and tracing how features_extractor, pi_features_extractor, and vf_features_extractor are used. Document when each extractor is active and add concrete network-output examples; the explanation should resolve whether shared and separate paths can be used together.

Written by the indexing model from the issue text.