Pooled output from DeBERTa(v2) model
### Feature request

Add a context pooler and include the `pooler_output` in the result of the `forward` method of the `(TF)Deberta(V2)Model`.

Changes required:
* introduce `add_pooling_layer` flag in the constructor
* add pooler
* change return type from `BaseModelOutput` to `BaseModelOutputWithPooling`
* update heads which currently do their own context pooling

### Motivation

* Simplification of use cases where the model is used to generate context embeddings (whether the embeddings being the end goal or only used as an input to additional layers in a custom model)
* Currently multiple heads require the pooled output and hence it is re-implemented in multiple places. The complexity of the heads and the redundancy can be decreased by moving this operation to the base model
* Consistency with BERT and RoBERTa implementations which already operate in this fashion. Additionally, this consistency can simplify use cases where the user works with context embeddings and wants to experiment with different models. E.g.: BERT and RoBERTa can be easily exchanged. On the other hand, swapping in DeBERTa requires the user to handle the context pooler

### Your contribution

I implemented the necessary changes in my fork and can submit a PR
Thanks for your suggestion! This will sadly be a breaking change as it will change the way the weights are named, and thus break compatibility with all DeBERTa models on the hub. So even if your suggestion makes a ton of sense, I don't think we will be able to implement it.<|||||>@sgugger thank you for your reply! I see why the implementation of the heads should not be altered, nevertheless, wouldn't it still make sense to at least add the pooler to the base model (the deberta model without any head)? Motivation being the first and the last points from the list above.

<|||||>We have plenty of other models (like ELECTRA) where the pooler is not implemented. As a general rule of thumb, it depends on whether it was in the pretraining objective or not.<|||||>I understand and thank you! I will close the issue then.