`accelerate` support for `RoBERTa` family
# What does this PR do?

This PR adds `accelerate` support for:
- `RoBERTa`
- `data2vec_text`
- `Lilt`
- `Luke`
- `XLM-RoBERTa`
- `CamemBERT`
- `LongFormer`
This way, any of the models above can be loaded in 8bit using `load_in_8bit=True`.

Since these models copy the same `xxxLMHead` from `RoBERTa` I had to change the copied modules too - happy also to break down this PR into several smaller PRs, 

This PR also fixes a small bug on `accelerate` tests where the variable `input_dict` is overriden by `xxForMultipleChoice` models. 

Can also confirm all slow tests pass (single + multiple GPUs)

cc @sgugger @ydshieh 

_The documentation is not available anymore as the PR was closed or merged._