ModelForSequenceClassification (Roberta/Bert) don't support mean_pool
# 🚀 Feature request

I hope I did not miss it, but it looks like there is no way to use `RobertaForSequenceClassification` or `BertForSequenceClassification` with mean_pool rather than `[CLS]` or `<s>` tokens as pooler. 

## Motivation

It would be useful for benchmarking and in some cases it can work better than the start of sequence token I believe?

## Your contribution

I can try to make the PR if it makes sense for the repo to add it
I think @patrickvonplaten might be the person to tag to check if this slipped through.

If this doesn't make sense I'll close the issue!<|||||>Think in this case the best is to do a little tweak to the original modeling file and use this one instead :-) <|||||>cc @sgugger - think that's also a good use case for: https://huggingface.co/docs/transformers/custom_models? <|||||>BERT and RoBERTa supports the pooling that was introduced in their paper. Transformers is not a modular toolbox, so as Patrick said, if you want to tweak this, you will need to edit the modeling file, but we won't accept a change in the code to support this. And as Patrick pointed out, you can still share your tweaked model if it performs better!<|||||>Ok thanks! I will close the issue then