use `BaseModelOutput` as common interface for all different `BaseModelOutputWith*`?
Hello team,

I have been taking a look at the `different` output models from your models, and I wonder if it would make sense to inherit all the `BaseModelOutputWithPool` and all the other flavours of modeling output, instead of using `ModelOutput`.

https://github.com/huggingface/transformers/blob/c301c26370dfa48f6a6d0408b5bb9eb70ca831b3/src/transformers/modeling_outputs.py#L24

We are trying to build a wrapper around many of the public models hosted on hugging face, and it would be useful to know if we can assume that all the potential `outputs` of the models will contain `hidden_states`. Since now they all only inherit from `ModelOutput`  it seems a little confusing.

Am I missing something? Is it not something that can be assumed? 