Map `RealmBertModel` for `AutoModel`
# What does this PR do?

This PR maps `RealmModel` to be able to use it with `AutoModel` - for consistency with `BertModel` 

Why this PR? I wanted to automate tests for `BetterTransformers` integration in `optimum` without having to import manually the class, see here: https://github.com/younesbelkada/optimum/blob/49575c5b016392383f0c2ebc1565ef56747b87e6/tests/bettertransformers/test_bettertransformers.py#L68 
Since `Realm` should be supported by `BetterTransformers`, this PR would help me design easier-to-implement tests

cc @sgugger @ydshieh 

https://github.com/huggingface/optimum/pull/423
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi @younesbelkada 
We should also work on `src/transformers/__init__.py`, but otherwise LGTM, thanks!<|||||>Ah yes, great catch! Will add it now<|||||>Thanks a lot for the explanation!
Yes let's stay pragmatic, I will probably just remove it from the `BetterTransformers` test <|||||>@sgugger But we usually expose the base model, no, like `BertModel`?<|||||>Yes, but I'm very unsure that this is the base REALM model. It's more of a building block towards it.