Add `ElectraForCausalLM` -> Enable Electra encoder-decoder model
# What does this PR do?

This PR adds `ElectraForCausalLM` and thus enable to use `Electra` model in the encoder-decoder setting.

Fixes #14269


## Before submitting
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

@patrickvonplaten 

**Additional context:** I've already tried to fine-tune Electra model as an encoder-decoder model on our use case and it proved to be working.
This looks good to me as well! Regarding the docs, could you try to follow @sgugger 's suggestion [here](https://github.com/huggingface/transformers/pull/13967#issuecomment-999565664) on how to fix it? 

Let me know if you need any help :-)<|||||>Merging as failing test has been fixed on master and other failures are unrelated. Thanks a lot for you work @stancld !