Add `ElectraForCausalLM` to enable constructing Electra-based `EncoderDecoderModel`
# 🚀 Feature request

Add `ElectraForCausalLM` model to enable constructing `EncoderDecoderModel` based on Electra.

## Motivation

Currently, there are many encoder-only models, such as BERT or RoBERTA, which have already been supported for encoder-decoder settings. Even though Electra is pre-trained in a different fashion, I found out the fine-tuning an Electra2Electra model works as well.
Also, there are publicly available checkpoints for a small configuration of the model and thus one can try to train a model which is an order of magnitude smaller than e.g. Bert2Bert or RoBERTa2RoBERTa.

## Your contribution

I have already prepared an implementation for myself. If there's a desire to add this module I'll be happy to work on the PR. :]