Support Roberta on `accelerate`
# What does this PR do?

This PR adds `Roberta` model family support with `accelerate` ! This aims to support `int8` quantization for these models.
Before merging I have notice few nits that I need to fix and discuss! I am unsure whether these fixes should be here or in `accelerate`.

1- If the model has the attribute `_keys_to_ignore_on_save`, it seems that it does not get properly initialized by `accelerate` (but I might be missing something here). AFAIK all models that has `accelerate` support for now, has at most the attribute `_keys_to_ignore_on_load_missing` but not `_keys_to_ignore_on_save`. Therefore [when the base model gets saved in the `accelerate` test](https://github.com/younesbelkada/transformers/blob/aefea27619b43cbb3e518ea972009e525676dc8d/tests/models/roberta/test_modeling_roberta.py#L587), these parameters does not get saved in the state_dict. I had to come up with a modification in the `_load_pretrained_model` function to randomly initialize these parameters since they're ignored by the `_load_state_dict_into_meta_model`. This post processing trick happens [here](https://github.com/younesbelkada/transformers/blob/aefea27619b43cbb3e518ea972009e525676dc8d/src/transformers/modeling_utils.py#L2598). 
2- Therefore I had to change the `accelerate` tests. Since the parameters that are assigned in the `_keys_to_ignore_on_save` are initialized randomly, I propose to check the logits compatibility between the base model and the accelerate model only for the attention outputs and not the `lm_head` output. These modifications happens [here](https://github.com/younesbelkada/transformers/blob/aefea27619b43cbb3e518ea972009e525676dc8d/tests/models/roberta/test_modeling_roberta.py#L569) - maybe this modification could happen in the super class?
3- Last nit: in the `accelerate` tests, it is better to not override the variable `inputs_dict` since inside the main loop, we can switch from a `xxxForMultipleChoice` to a `xxxForQuestionAnswering` model. Therefore, this variable does not get modified by the class function `_prepare_for_class` since it [gets modified only if the model is a `MODEL_FOR_MULTIPLE_CHOICE` model.](https://github.com/younesbelkada/transformers/blob/aefea27619b43cbb3e518ea972009e525676dc8d/tests/test_modeling_common.py#L165-L173) and not reset to the correct `inputs_dict` afterwards.

Need also to fix the slow test for `lilt` model where I am having `ValueError: embeddings.position_ids doesn't have any device set.`

cc @sgugger  

_The documentation is not available anymore as the PR was closed or merged._<|||||>Closing in favor of #19906