Make ElectraPreTrainedModel importable
# 🚀 Feature request

Make `ElectraPreTrainedModel` importable

## Motivation

Consistency.

`DistilBertPreTrainedModel` and `AlbertPreTrainedModel` are importable but I cannot import `ElectraPretrainedModel` from v2.8.0
(see https://github.com/huggingface/transformers/issues/1968)

## Your contribution

https://github.com/huggingface/transformers/pull/4173

Thanks for contributing #4173 :).